Heat Exchange Element and Process for Production

ABSTRACT

The invention provides a heat exchange element comprising a substrate and a coating, wherein the coating is present on at least a part of a flow path defined by the heat exchange element. The coating comprises a metal and has a structure comprising spikes having a length of up to 100 μm; the average length of the spikes various throughout the coating. The invention also provides a method of transferring heat to or from a fluid which comprises providing the fluid to a flow path of the heat exchange element of the invention. The invention further provides a process for producing a heat exchange element of the invention, wherein the process comprises providing an electroless deposition solution to a surface of a substrate. The invention further provides a flow process for producing a heat exchange element and a heat exchange element obtained or obtainable by that process.

FIELD OF THE INVENTION

The present invention provides a heat exchange element which facilitatesefficient heat transfer. The invention also provides a method oftransferring heat to or from a fluid using a heat exchange element ofthe invention. Processes suitable for producing heat exchange elements,including the heat exchange element of the invention, are also provided,in particular an electroless flow deposition process. The invention alsoprovides a heat exchange element obtained or obtainable by theelectroless flow deposition process.

BACKGROUND TO THE INVENTION

The transfer of heat across surfaces is of importance in many productsand systems. Examples of such systems include cooling systems (such asair conditioning systems or refrigeration systems) and heating systemssuch as boilers. Other examples of such systems include heat recoverysystems. A typical configuration of an apparatus for heat exchange insuch a system involves the transfer of heat between a heat exchangeelement and a fluid in contact with the surface of that element. A widevariety of sources may be used to provide heat to the heat exchangeelement. Examples of such configurations include, for example, heatexchangers (where the source of heat to the heat exchanger is a secondfluid in contact with the reverse side of the heat exchanger element),boilers, radiators, refrigerators and so on.

It is therefore desirable to provide a heat exchange element which hasvery good heat transfer properties. It is particularly desirable toprovide a heat exchange element which can efficiently transfer heat to afluid such as a liquid in contact with the said element. However, theprocesses involved in heat transfer across a surface, and particularlyfrom a solid surface to a liquid, are complex and poorly understood. Itis therefore not a straightforward matter to produce a heat exchangeelement which has good heat transfer properties, or to optimise existingsurfaces to improve their heat transfer properties.

Previous efforts have been made in this field. The approach taken hastypically involved maximising surface area of an object intended for usein heat transfer.

Some previous workers have attempted to control the heat transferability of an object by providing its surface with a specificwettability. In one example, WO 2011/149494 describes a heat exchangesurface having a preselected contact angle with a particular liquid. Thesurface is produced by providing hydrophilic nanostructures on asubstrate. The surface nanostructure is formed by depositing oxide-basednanomaterials on a substrate, and the nanostructures have an averageroot-mean-square roughness, or height, of 200 to 600 nm. A surface thusproduced is said to be useful in pool boiling experiments.

Other previous workers have attempted to control the heat transferability of a surface by providing precisely engineered structuresthereon. The engineered structures are usually made of silicon. Anexample is found in “Surface structure enhanced microchannel flowboiling”, Zhu et al., Journal of Heat Transfer, Vol. 138, pp 091501-1 to091501-13. A microchannel is provided having an array of siliconmicropillars, and is said to promote heat transfer in a flow boilingregime.

The present inventors have previously provided a nano-rough surfacehaving a hierarchical nanostructure for use in heat transfer, describedin WO2014/064450. It was found that, by carrying out electrolessdeposition for a limited period of time, a coating having a hierarchicalnanostructure could be produced on a substrate. Typically, thehierarchical nanostructure comprised a first-level structure coated witha second level of structure of ten or one hundred times smaller size.Typically, the first-level structures were up to 500 nm high and thesecond level of structure comprised features up to 50 nm high. Thesesurfaces were shown to effect heat transfer in flow boiling experiments.

It is an object of the invention to provide a heat exchange elementhaving heat transfer properties comparable to or better than the heatexchange elements discussed above, and that is suitable for heattransfer to a wide range of fluids. Improved heat exchange propertiesallow a heat exchange element of the invention to cool a heat sourcefaster and via a smaller element, saving space and weight.

In addition to providing a heat exchange element with good heat transferproperties, it is an object of the invention to provide a process forproducing a heat exchange element. Many known methods of producing heatexchange elements are laborious and costly. These known methods arecommonly methods for increasing the surface area of an object intendedfor use in heat transfer

One previous example of a method of producing a heat transfer surface isdescribed in “Surface structure enhanced microchannel flow boiling”, Zhuet al., Journal of Heat Transfer, Vol. 138, pp 091501-1 to 091501-13. Inthat method, engineering of silicon structures on a surface was used toincrease the surface area of an object. The methods used to engineer thesilicon structures include ion etching of a silicon substrate andbonding of a silicon wafer to a silicon surface.

The present inventors have previously described the use of electrolessdeposition to create a coating on a heat exchange element(WO2014/064450). In that case, the electroless deposition processcreated a nano-rough surface and the method involved placing a substratein a bath of electroless deposition solution. However, the electrolessdeposition of a metal in a bath process suffered from a bubble adhesionissue. Electroless deposition of a metal on a substrate usually producesbubbles of hydrogen gas at the surface of the substrate. It was foundthat, during the bath process, bubbles of hydrogen produced duringelectroless deposition stuck to the substrate surface and caused thecoating to form around the bubbles. This had two particular adverseeffects. Firstly, the rough structure of the coating formed by theelectroless deposition process was disrupted by the presence of bubbles,causing gaps in the coating and/or portions of the coating not havingthe desired rough structure. Secondly, the coating formed by electrolessdeposition was formed over the bubbles, leading to portions of thecoating not in contact with the substrate. These non-adhered portions ofthe coating were found to be fragile and frequently peeled away from thesubstrate over time, for instance during use of the coated substrate inheat exchange. This was undesirable as it reduced the heat exchangeefficacy of the coating. Moreover, the exposed portions of the substratecaused by the hydrogen bubbles were subject to corrosion during use ofthe coated object as a heat exchange element.

Electroless deposition methods remain desirable for producing heatexchange coatings as they can produce rough structures at lowtemperature, which reduces the cost of the process. Electrolessdeposition is also desirable as it can be used to provide a coatingcontaining metal, and hence having good heat exchange properties, whichis desirable in a coating for a heat exchange element. Moreover,electroless deposition processes require less material than hot dipgalvanising processes (which are also referred to as galvanic depositionprocesses) which are performed in a bath of liquid metal.

It is an object of the present invention to provide a process which canbe performed cheaply and quickly, ideally at a low temperature tominimise energy costs. It is also desired to provide a process which canbe performed on an existing heat exchanger in situ, so that the processmay advantageously be used to retro-fit the heat exchange element of theinvention in an existing heat exchanger. Further, it is an object of theinvention to provide a process for providing a coating suitable for aheat exchange element (that is, a rough coating comprising a metal) to asubstrate, said process having the advantages of an electrolessdeposition process but avoiding the above-mentioned difficulties.

SUMMARY OF THE INVENTION

The inventors have found that a heat exchange element having a coatingcomprising sharp spikes in the micrometre size range, wherein the lengthof the spikes varies over a surface of the heat exchange element, hasparticularly advantageous heat transfer properties. The inventiontherefore provides a heat exchange element comprising a substrate and acoating, wherein the heat exchange element defines a flow path for flowof fluid, and wherein at least a part of the flow path is coated withthe coating, wherein:

-   -   the coating comprises a metal;    -   the coating comprises a plurality of spikes of a length of up to        100 μm;    -   the coating comprises a first region at an end of the flow path        in which the average spike length is S₁ and a second region on        the flow path in which the average spike length is S₂; and    -   S₁ is greater than S₂.

The heat exchange element of the invention is particularly well suitedto promoting efficient heat transfer between the spiky surface and afluid. Accordingly, the invention provides a method of transferring heatto or from a fluid which comprises passing the fluid along a flow pathof a heat exchange element.

A coating as comprised in the heat exchange element of the invention canconveniently be formed by electroless deposition. The inventiontherefore further provides a process for producing a heat exchangeelement of the invention wherein the process comprises providing anelectroless deposition solution to a surface of a substrate.

The inventors have further surprisingly found that flowing anelectroless deposition solution comprising a metal ion over a substrateremoves hydrogen bubbles rapidly from the surface and hence reduces theissues of fragility and/or peeling of the deposited coating which areassociated with the bath processes. Moreover, the flow of electrolessdeposition solution unexpectedly still provides a rough surface suitablefor promoting heat transfer from the surface. This finding is unexpectedas it was previously thought that the flow process would lead to anirregular structure of the coating, that would not have such good heatexchange properties. Moreover, this electroless flow deposition processis found to be capable of producing a heat exchange element havingregions of differing spike length according to an embodiment of the heatexchange element of the invention. Particularly advantageously, thiselectroless flow deposition process may be used to retrofit anelectrolessly deposited coating to an existing heat exchanger in situand without disassembly. The deposition solution can be provided only tothose parts of a heat exchanger which require coating, thus minimisingwaste of material.

The invention therefore provides a process for producing a heat exchangeelement comprising a substrate and a coating, wherein:

-   -   the coating comprises a metal; and    -   the process comprises flowing an electroless deposition solution        over a surface of the substrate.

This electroless flow deposition process produces an object having arough coating, the coating comprising a metal. The coating comprising ametal is a good conductor of heat and has a large surface area, whichincreases contact between the heat exchange element and thesurroundings, promoting heat transfer between the element and thesurroundings. Therefore the heat exchange element produced by thisprocess is suitable for use in heat exchange.

The invention also provides a heat exchange element obtained orobtainable by this electroless flow deposition process.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates the different modes of heat transfer from a surfaceto a fluid.

FIG. 2 illustrates diagrammatically the coating on the heat exchangeelement of the invention. FIG. 2a shows model spikes in differingorientations. FIG. 2b shows a coating (1) on a substrate (2). FIG. 2cshows an arrangement of clusters on a surface and the pore therebetween. FIG. 2d shows a coating (1) on a substrate (2), the coatinghaving a graduating spike length across the substrate surface. FIG. 2eillustrates diagrammatically a heat exchange element comprising acoating (1) on a substrate (2), the coating formed on a substrate by theelectroless flow process of the invention. FIG. 2f shows a cross-sectionof a heat exchange element (5) comprising a coated flow channel and anuncoated flow channel.

FIG. 3 contains SEM images of coatings according to the invention. FIG.3(a) shows a coating comprising spikes of 1 to 3 μm in length; FIG. 3(b)shows a coating comprising spikes of 4 to 5 μm in length; and FIG. 3(c)shows a coating comprising spikes of 8 to 10 μm in length.

FIG. 4 is an SEM image of a coating applied to the inside of a heatexchanger, comprising spikes which are approximately 3 μm in length.This coating was produced by the electroless flow deposition process ofthe invention.

FIG. 5 contains SEM images of coatings according to the inventionwherein the coatings comprise clusters. FIG. 5(a) shows a coatingcomprising spikes of approximately 7 μm in length arranged in clusters.FIG. 5(b) also shows a coating of spikes arranged in clusters, at lowerresolution.

FIG. 6 is an SEM image of a coating according to the inventioncomprising spikes approximately 7 μm in length applied to a 75μm-diameter wire mesh.

FIGS. 7 and 8 show the heat flux in kW m⁻² from a surface to an organicrefrigerant as a function of wall superheat (ΔT_(c)) for variousdifferent surfaces. In FIG. 7, one surface is a polished surface and theother is a surface coated according to the electroless flow depositionprocess defined herein.

FIG. 9 shows the heat transfer coefficient in W m⁻² K⁻¹ for a surfacecoated according to the invention (by the electroless flow depositionprocess), and an uncoated surface, at a variety of refrigerant flowrates.

FIG. 10 shows the approximate spike height (dashed upper line) and spikebase radius (solid lower line) achieved by an electroless flowdeposition process according to the invention over time.

FIG. 11 shows a test rig used to compare the heat exchange performanceof an evaporator (heat exchanger) to that of an uncoated evaporator. Thecoated evaporator was coated in accordance with the electroless flowdeposition process of the invention to provide a heat exchange elementaccording to the invention.

FIG. 12 shows the heat exchange coefficients as a function of heattransfer rate for the coated and uncoated evaporators tested in the rigshown in FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION Heat Transfer Across a Surface

In the accompanying drawings FIG. 1 (upper image) illustrates how theheat transfer across a surface into a fluid (in this case, a liquid)varies with temperature. The Figure illustrates the several differentmodes of heat transfer available. At lower temperatures heat transferfrom a hot (e.g. metal) surface to a fluid (e.g. water) works wellthrough natural convection, especially if there is complete wetting ofthe surface by the fluid. As the temperature of the heat transfersurface increases, the formation of bubble nuclei leads to heat transferoccurring by nucleate boiling. The temperature at which the onset ofsuch nucleate boiling occurs is affected by surface roughness and theexistence of regions that are strongly hydrophobic. As temperatureincreases further there is a transition to stable film boiling. Instable film boiling a layer of vapour exists next to the surface andheat transfer through this film is thought to occur by conduction. Inthe transition from nucleate boiling to stable film boiling some areasof the surface display film boiling and some nucleate boiling. Becausethe thermal conductivity of the vapour is lower than the liquid, theheat flux across the surface tends to reduce in the transition boilingregion before reaching a minimum at the onset of stable film boiling,and then increasing again with temperature. The lower image of FIG. 1also illustrates how the heat flux from the surface into the fluid(q_(w)) varies with the surface superheat (ΔT_(c), which is thedifference in temperature between the temperature of the surface and thetemperature of the fluid).

Without wishing to be bound by theory, it is speculated that theadvantages of the heat exchange element of the invention may beattributable to various aspects of the structure of the coating.

It is believed that in the nucleate boiling regime, an important factorcontributing to the efficiency of heat transfer is the sharpness of thespikes in the coating. It is speculated that the sharper the spikes, themore efficiently the processes required for heat transfer by nucleateboiling occur, including:

-   -   formation of a bubble at a spike tip;    -   transfer of the bubble down the side of the spike into a cavity        or pore;    -   growth of the bubble in a cavity or pore by addition of vapour;        and    -   detachment of the bubble and re-wetting of the surface.

The spikes at the surface of the heat exchange element of the inventionare sharp, and hence promote the efficient performance of the abovesteps and hence of heat transfer. Further, the spike concentrates theflow of heat to the tip of the spike, promoting the above processes.

Further preferred features of the coating also contribute toimprovements in heat transfer, in particular in the nucleate boilingregime. These include the size and shape of the spikes, the density ofthe spikes, the pore or cavity sizes on the surface and the density ofpores/cavities (pores or cavities are spaces between spikes; these arediscussed in more detail below). The surface is believed to have anadvantageous balance between the presence of enough spikes to createbubbles and the presence of enough pores/cavities to store and growbubbles. Bubble nucleation is believed to occur at or near the tips ofspikes and so the high density of spikes in the coating advantageouslyprovides a large number of nucleation sites, enabling the efficientcreation of bubbles. During bubble growth, heat is transferred to thebubble by boiling of liquid to produce the gas in the bubble. Bubblegrowth is advantageous for heat transfer and is promoted by theexistence of bubble growth sites such as pores and cavities.

The coating of the invention has a density of pores/cavities largeenough to promote bubble growth, but not so large that the number ofbubble nucleation sites is compromised. The size of the pores/cavitiesin the coating of the invention is also believed to be suitable forpromoting efficient bubble growth. The surface is also easilyre-wettable to detach bubbles from the surface easily. Control of thesize of the spikes, and the size of the pores or cavities between thespikes, is therefore believed to be advantageous in promoting heattransfer by a boiling regime.

The heat exchange element of the invention provides variation in thelength of the spikes at different regions on a flow path defined by thesaid element. A flow path is a route along which a fluid can flow over asurface of the heat exchange element. In one region, at an end of theflow path, the average spike length is longer than in another region ata different point on the flow path. The different regions are believedto be suited to different kinds of heat transfer. The first region,having a longer average spike length and therefore deepercavities/pores, is better suited to promoting nucleate boiling of afluid passing along the flow path of the heat exchange element. Thesecond region is less suited to promoting nucleate boiling and moresuited to promoting film boiling of a fluid passing along the flow pathof the heat exchange element. Thus, the heat exchange element of theinvention advantageously provides regions suited to at least two kindsof heat transfer regime.

It is further speculated that the arrangement of these regions along theflow path of the heat exchange element of the invention assists inpromoting heat transfer during flow boiling. The region of longer spikesis beneficially arranged at an end of the flow path, where a fluid to becooled may commence its flow along the flow path. Nucleate boiling maybe initiated in this region. As the fluid flows along the flow path, itencounters the second region of shorter spikes which may assist theestablishment of an efficient film boiling regime further along the flowpath.

Heat Exchange Element

It should be noted herein that reference to the “heat exchange elementof the invention” indicates a heat exchange element as defined in claim1. Reference to a heat exchange element indicates a heat exchangeelement which can be formed according to the electroless depositionprocess of the invention. In preferred embodiments, the heat exchangeelement comprises all the features of claim 1 and is a heat exchangeelement of the invention.

By “heat exchange element” is meant a solid object suitable fortransferring heat from itself to its surroundings. The surroundings maybe, for example a solid or fluid adjacent to (and usually in contactwith) the heat exchange element. The heat exchange element is capable ofabsorbing heat from a source. The heat source may be, for example, asolid or fluid adjacent to (and usually in contact with) the heatexchange element. Thus, the heat exchange element of the invention is asolid object capable of transferring heat from a source, via itself, toits surroundings. In particular, the heat exchange element of theinvention is suitable for transferring heat to a fluid, particularly toa liquid because a liquid is capable of boiling.

The heat exchange element of the invention defines a flow path for flowof fluid. A heat exchange element which is not a heat exchange elementof the invention may also define a flow path for fluid. The flow pathfor flow of fluid, also referred to as a flow path, is a route alongwhich a fluid can flow over a surface of the heat exchange element.Thus, the flow path comprises at least part of an exposed surface of theheat exchange element. An exposed surface of the heat exchange elementis one which may come into direct contact with the surroundings. Theflow path must be exposed in order that fluid may come into contact withit. For example, where the heat exchange element is in the form of aplate, the flow path may be any route over a surface of the plate. Insome embodiments, the flow path may pass through the heat exchangeelement. For instance, the heat exchange element may comprise one ormore channel(s) (including an open channel or a closed channel, i.e. atube) to allow fluid to flow through the element; in that case, the flowpath may include at least a part of the one or more channel(s) throughthe heat exchange element.

The flow path is typically all of or part of the transfer area of theheat exchange element. By “transfer area” is meant the area of the heatexchange element which may contact a fluid to which heat is to betransferred (such a fluid is referred to as a working fluid or arefrigerant). In some embodiments, the flow path is the whole surfacearea of the heat exchange element which can contact a fluid to whichheat is to be transferred (a refrigerant). In other embodiments, theflow path may comprise only part of the surface area of the heatexchange element which can contact a fluid to which heat is to betransferred. Where the heat exchange element comprises a channel (orflow channel) for carrying the fluid to which heat is to be transferred,in some embodiments the flow path comprises part of the surface of theflow channel. In other embodiments the flow path comprises all of thesurface of the flow channel.

Where the heat exchange element comprises a flow channel, an end of theflow path may be situated at an end of the flow channel. For instance,an end of the flow path may be situated at an inlet to the flow channel.Alternatively or additionally, an end of the flow path may be situatedwithin a flow channel (not at its end), e.g. distant from an inlet tothe flow channel. Typically, an end of the flow path is at a position onthe heat exchanger which is first contacted by a fluid to which heat isto be transferred. Typically, an end of the flow path is situated wherethe fluid to which heat is to be transferred first comes into contactwith the coating described herein.

A cross-section of a heat exchange element (5) is illustrated in FIG. 2f. The heat exchange element (5) has a first flow channel and a secondflow channel, each having an inlet (3). The first flow channel comprisesa coating (1) while the substrate (2) is not coated in the second flowchannel. The first flow channel defines a flow path (4) therethrough.

The flow path is usually continuous, meaning that it is a path alongwhich fluid can flow while in continuous contact with the heat exchangeelement. The flow path may comprise more than one surface of the heatexchange element, e.g. an internal surface and an external surface.

It should be noted that a flow path does not necessarily constitute apath along which fluid is directed to flow. Rather, the flow pathconstitutes at least part of an exposed surface of the heat exchangeelement that fluid can contact with, and hence that fluid can flow over.

The heat exchange element (e.g. the heat exchange element of theinvention) may be incorporated into a product. In one embodiment of theinvention, the heat exchange element is incorporated into a heatexchanger. In another embodiment of the invention, the heat exchangeelement is incorporated into an air conditioning unit, refrigerator,heat recovery system, radiator, heat sink, solar collector, boiler, orheat exchanger such as a mini heat exchanger or microchannel heatexchanger.

Coating

The heat exchange element comprises a coating which promotes heattransfer from the heat exchange element. The heat exchange element istherefore able to transfer heat to its surroundings efficiently via thecoating. Thus, typically the coating of the heat exchange element ispresent on a heat transfer surface of the heat exchange element. A heattransfer surface is a surface of the heat exchange element suitable fortransferring heat to the surroundings. A heat transfer surface maycontact the surroundings directly or indirectly; for example, a heattransfer surface may have one or more layers thereon which separate itfrom direct contact with the surroundings. Typically, the coating ispresent directly on the heat transfer surface.

An exposed surface of the heat exchange element is a surface which isexposed to the surroundings. Typically, at least a part of the coatingis an exposed surface. However, in some embodiments a further layer ispresent on the coating such that the further layer(s) form the exposedsurface.

In the heat exchange element of the invention, at least a part of theflow path is coated with the coating. In some embodiments, the entireflow path is coated with the coating. This embodiment may be preferredas it enables the heat exchange element to maximise heat transfer to afluid in contact with, e.g. flowing along, the flow path.

The coating may also be present elsewhere on the heat exchanger (i.e.other than along the flow path). Advantageously, the coating may bepresent only on those parts of the heat exchange element which are fortransferring heat to a fluid. This minimises waste of material.

The coating may be a continuous coating, wherein the entire coatedportion or portions of the substrate are covered with the coating.Alternatively, the coating may be discontinuous, such that there aregaps in the coating on the coated portion of the substrate where thecoating is not present on the substrate. Preferably, the coating is acontinuous coating, as illustrated in FIG. 2 b.

In the heat exchange element of the invention, the coating comprisesspikes. However, not all of the material of the coating is necessarilyarranged in spikes; the coating may also comprise material distributedover the substrate surface. Thus a continuous coating does not requirespikes to be arranged side-by-side without gaps.

In the heat exchange element of the invention, the coating comprises aplurality of spikes. By “spike” is meant a structure having a thickerportion at one the end of the structure which tapers to a thinnerportion at the other end of the structure. A spike may therefore bedescribed as a structure having a pointed top, or as a taperingstructure. The thickest part of the spike (base) is typically at the endof the spike nearest to the substrate and the thinnest part of thestructure (tip) is typically at the end of the spike furthest from thesubstrate.

The base of the spike is defined as the smallest cross-section of thespike which intersects the end of the shortest side of the spike. Forinstance, when a spike extends at right angles to a flat surface, allsides of the spike (that is, distances from one end of the spike to theother) will have the same length and the tip of the spike lies along aline forming a right angle with the base. The base of the spike istherefore the cross-section of the spike at its plane of contact withthe substrate. This is illustrated in the left-hand image of FIG. 2 a.However, when a spike extends at 45° from a flat surface, its tip maynot lie over the base at all. In that case, the length of the sides ofthe spike as measured from the substrate to the spike tip will varydepending on where they are measured. The base of the spike is thereforepositioned at the base of the shortest side and tilted at 45° to thebase (right-hand image of FIG. 2a ).

A spike is usually approximately cone-shaped. That is, the spike may beapproximated to a cone having a circular base and a tip lying along anaxis extending at right angles to the base. The approximately circularbase is taken to be the smallest circle which contains the true base.

-   -   The length of a spike is taken to be the distance from the        centre of the circular base to the spike tip in this        approximated cone shape.    -   The base radius of a spike is the radius of the approximated        circular base.    -   The cone angle is the angle the sides of the cone make with its        central axis, measured at the spike tip.

The coating comprises a plurality of spikes of a length of up to 100 μm.In general, the coating comprises spikes having a length of at least 1μm. The coating also generally comprises spikes having a length of up to50 μm. Thus, typically the coating comprises a plurality of spikeshaving a length of at least 1 μm and no more than 50 μm. Preferably, thespikes have a length of 1 to 15 μm, e.g. 2 to 10 μm, for example 3, 4,5, 6 or 7 μm.

The cone angle of the spikes in the plurality of spikes is typicallysmall. The cone angle is generally less than 40°. In some embodiments,the cone angle is from 2° to 30°, e.g. approximately 5° or approximately10° or approximately 20°. In the context of cone angle, an approximatevalue may vary by ±5°, for instance ±2°.

The cone angle of the spikes stays within the above-specified rangesregardless of spike length. Accordingly, the spike base radius of thespikes in the plurality of spikes increases with spike length.Typically, the spike base radius is less than 5 μm. For example, thespike base radius may be from 0.05 μm to 3 μm, preferably from 0.1 μm to2 μm or from 0.2 μm to 1 μm. In some embodiments, the spike length isfrom 1 to 15 μm and the spike base radius is from 0.2 to 3 μm. In someembodiments, the spike length is from 1 to 10 μm and the spike baseradius is from 0.1 to 2 μm. In some embodiments, the spike length isfrom 2 to 10 μm and the spike radius is from 0.2 μm to 1 μm.

The spike length, cone angle and spike base radius may all be calculatedby taking an SEM image of the coating and fitting approximate cones tothe spike or spikes observed in that image. The fitting may be done byeye or by computer modelling.

The spikes are generally arranged close to one another. The density ofspikes in the coating (that is, the number of spikes per unit area) willusually vary with the spike base radius, as a smaller base radius willallow spikes to be more closely packed. In general, the coatingcomprises 5 or more spikes per 100 μm², e.g. at least 10 spikes per 100μm², or at least 20 spikes per 100 μm^(2.) The base radius of the spikesalso imposes an approximate upper limit on the density of spikes;however, where the spikes are arranged in clusters (see below) thedensity of spikes may be increased. Thus, in general the coatingcomprises no more than 500 spikes per 100 μm², e.g. no more than 200spikes per 100 μm². Preferably, the coating comprises from 5 to 500,e.g. from 5 to 200, spikes per 100 μm².

The coating comprises a first region wherein the average spike length isS₁ and a second region wherein the average spike length is S₂. By“average spike length” is meant “mean spike length”. Average spikelength may be calculated by establishing the length of each spike in aregion and calculating the mean therefrom. More conveniently, averagespike length may be calculated on the basis of a representative sampleof spikes in a region. S₁ and S₂ may take values up to up to 100 μm. S₁and S₂ are generally at least 1 μm. Also, S₁ and S₂ are generally 50 μmor less. Thus, typically S₁ and S₂ are at least 1 μm and no more than 50μm. Preferably, S₁ and S₂ are from 1 to 15 μm, e.g. 2 to 12 or 2 to 10μm.

By “region” is meant an area of the coating on the surface of thesubstrate. A region is typically an area of at least 20 μm², for examplean area of at least 50 μm².

The average spike length in the first region (S₁) is greater than theaverage spike length in the second region (S₂). Typically, S₂ is 95% ofS₁ or less. In some embodiments, S₂ is 90% of S₁ or less, e.g. 80% of S₂or less. Usually, S₂ is at least 10% of S₁, for instance at least 40% ofS₁. For example, S₂ may be from 10% of S₁ to 95% of S₁ or from 50% to90% of S₁.

S₁ may be up to 100 μm. Generally, S₁ is from 1 to 50 μm. In a preferredembodiment, S₁ is from 1 to 20 μm, e.g. from 2 to 15 μm. S₂ may be up to100 μm. Generally, S₂ is from 0.1 to 50 μm. In a preferred embodiment,S₂ is from 0.2 to 10 μm, e.g. from 0.5 to 10 μm. In one embodiment, S₁is from 1 to 20 μm and S₂ is from 0.2 to 12 μm, e.g. S₁ is from 2 to 12μm and S₂ is from 0.5 to 10 μm. In some embodiments, the differencebetween S₁ and S₂ is 0.1 μm or more, e.g. 0.5 μm or more or 1 μm ormore. For instance, the difference between S₁ and S₂ may be from 0.1 to5 μm.

The first region is located at an end of a flow path defined by the heatexchange element. For instance, where the flow path comprises a flowchannel (e.g. a tube) through a heat exchanger, the first region may belocated at the end of that flow channel. However, in some embodimentsthe flow path may not begin precisely at the end of such a flow channeland so the first region may be located some way inside the flow channel.The second region is located elsewhere on the flow path to the firstregion. For instance, the second region may be located towards thecentre of a flow channel.

The first and second regions may be isolated from one another. Forinstance, in a heat exchange element of the invention the first andsecond regions may be located on different plates or fins, or on indifferent flow channels. That is, in some embodiments of the invention,the first and second regions may exist in unconnected portions of thecoating. In other embodiments of the invention, the first and secondregions may be located in the same portion of coating.

In a particular embodiment, the heat exchange element of the inventioncomprises a coating wherein the coating comprises a first region at anend of the flow path in which the average spike length is S₁ and whereinthe average spike length decreases along at least a part of the flowpath, starting at the first region. In this embodiment, the secondregion may be any region (other than the first) along the said part ofthe flow path. In a preferred aspect of this embodiment, the averagespike length is graduated along at least a part of the flow path suchthat the longest average spike length occurs at the end of the flowpath, and the average spike length decreases along the flow path awayfrom that end. This aspect is illustrated in FIG. 2 d. A coatingaccording to this aspect of the invention may conveniently be achievedby the method of the invention. For instance, the time for which asubstrate is exposed to an electroless deposition solution may be variedalong the flow path by very slowly dipping the substrate into thesolution. Alternatively, an electroless deposition solution having aconcentration gradient that varies along the flow path may be providedto the substrate.

Thus, in one embodiment, the average spike length is graduated along allor part of the flow path. By “graduated” is meant that the average spikelength shows a change in a constant direction, i.e. a gradual orincremental rather than step-change. For example, the average spikelength measured at a series of neighbouring positions along the flowpath may be successively larger at each position. In one aspect of thisembodiment, the average spike length increases from one end of the flowpath to another. Where a flow path is coincident with a flow channel orpart of a flow channel, the average spike length may increase from oneend of the flow channel to another.

The coating of the heat exchange element of the invention may comprise athird region in which the average spike length is S₃. The third regionmay in one example lie on the flow path defined by the heat exchangeelement. S₃ may be up to 100 μm. Generally, S₃ is from 1 to 50 μm.Preferably, S₃ is from 1 to 20 μm, e.g. from 1 to 15 μm, e.g. 2 to 12 or2 to 10 μm.

The average spike length in the third region (S₃) is typically of asimilar order of magnitude to the first region (S₁) and greater than theaverage spike length in the second region (S₂). Typically, S₂ is 95% ofS₃ or less. In some embodiments, S₂ is 90% of S₃ or less, e.g. 80% of S₃or less. Usually, S₂ is at least 10% of S₃, for instance at least 40% ofS₃. For example, S₂ may be from 10% of S₃ to 95% of S₃ or from 50% to90% of S₃. S₃ is typically from 95 to 105%, for example from 99 to 101%,of S₁.

In one embodiment, the third region is located at an end of the flowpath. In this embodiment, the coating comprises a first region at an endof the flow path in which the average spike length is S₁, a secondregion on the flow path in which the average spike length is S₂, and athird region at another end of the flow path in which the average spikelength is S₁, wherein S₁ is greater than S₂. S₁ and S₂ may be as definedabove. In this embodiment, the average spike length shows a graduateddecrease and a graduated increase along the flow path.

The plurality of spikes comprised in the coating have sharp tips. Asexplained above, the sharpness of the spikes promotes nucleate boiling.Thus, the spikes are usually thin at the tip. Generally, a thickness atthe tip of the spikes is 100 nm or less. This means that the maximumdiameter of the spike, which occurs at the end of the spike (and at thetip of the approximate cone) is 100 nm or less. For instance thethickness at the tip of the spikes may be from 0.1 to 100 nm. Preferablythe thickness at the tip of the spikes is 60 nm or less. For example thethickness at the tip of the spikes may be from 1 to 50 nm.

In some embodiments of the invention, the spikes are arranged inclusters. The invention therefore provides a heat exchange elementcomprising a substrate and a coating as described herein, wherein thecoating comprises a plurality of spikes arranged in one or moreclusters. Each cluster comprises two or more spikes. Particularly goodheat transfer efficiencies have been observed using heat exchangeelements comprising clusters.

The number of spikes above two in a cluster is not particularly limited.Preferably, a cluster comprises five or more spikes. Typically, acluster comprises between 5 and 500 spikes.

A cluster is typically a flower-like arrangement of spikes. A clustercomprises two or more spikes protruding from a node. A node is a volumeof coating material from which the spikes of that cluster protrudeoutward. The node may be approximately spherical or hemispherical inshape. The spikes protrude from the node in an approximately radialfashion (that is, approximately in directions along the radii of aspherical or hemispherical node). However, significant deviations inradial orientation are possible and so each spike may not lie perfectlyalong the radius of the sphere or hemisphere. A cluster and node areillustrated in FIG. 2 c.

The node may usually have a diameter (corresponding to a diameter of asphere or hemisphere approximating to the node) of up to 50 μm.Generally the diameter of the node is from 0.05 to 50 μm, e.g. from 0.1to 20 μm, or 0.5 to 10 μm. Where the node is very small, it may be moreconveniently thought of as a point from which the spikes protrude.

A cluster may be described as having a height and a diameter. The heightis the longest distance perpendicular to the substrate. The diameter isthe diameter of the smallest circle in the plane of the substrate whichencloses the cluster when viewed from perpendicularly above the cluster.The height and diameter of a cluster may be determined by taking an SEMimage of the coating comprising the cluster and fitting the height anddiameter thereto, either by eye or via computer modelling.

The diameter of a cluster is generally less than 200 μm. The diameter ofa cluster is typically from 1 to 200 μm. Preferably, the diameter of acluster is from 2 to 100 μm, more preferably from 5 to 50 μm or 10 to 50μm, e.g. from 10 to 40 μm.

The height of a cluster is generally less than 200 μm. The height of acluster is typically from 0.5 to 150 μm. Preferably, the height of acluster is from 1 to 100 μm, more preferably from 2 to 50 μm, e.g. from5 to 30 μm.

The density of clusters (that is, the number of clusters per unit area)will vary with cluster diameter. Where the coating comprises clusters,the density of clusters is generally up to 100 clusters per 100 μm².Preferably, the density of clusters is from 0.5 to 50 clusters per 100μm², e.g. from 1 to 25 clusters per 100 μm².

Where the coating comprises clusters, the first region may comprise agreater density of clusters than the second region. Similarly, where thecoating comprises clusters, the average diameter of the clusters in thefirst region may be larger than the average diameter of clusters in thesecond region. Average diameter in this context means mean diameter. Theaverage cluster diameter may be calculated by establishing the diameterin a region and calculating the mean therefrom. More conveniently,average cluster diameter may be calculated on the basis of arepresentative sample of clusters in a region.

The presence of spikes and/or clusters in a coating gives rise tocavities and pores. By “pore” is meant a space between adjacentclusters; by “cavity” is meant a space between adjacent spikes. Cavitiesare therefore typically smaller than pores. The shape of a pore is notparticularly limited. A pore may be described as having a depth, a widthand a length. The depth of the pore is the maximum distance (in adirection perpendicular to the substrate) from the part of the poreclosest to the substrate to the maximum height of a neighbouringcluster. The length of the pore is the greatest straight linear extentof the pore in the plane of the substrate surface. The width of the poreis the greatest straight linear extent of the pore perpendicular to itslength in the plane of the pore. As with the other characteristics ofthe surface, these parameters may be determined by taking an SEM imageof the coating and fitting the parameters, either by eye or via computermodelling.

Generally, the depth of a pore is approximately equivalent to the heightof the adjoining clusters. Thus, the depth of a pore is generally lessthan 200 μm. The depth of a pore is typically from 0.5 to 150 μm.Preferably, the depth of a pore is from 1 to 100 μm, more preferablyfrom 2 to 50 μm, e.g. from 5 to 30 μm.

Generally, the length of a pore is less than 500 μm. The length of apore is typically from 2 to 250 μm. Preferably the length of a pore isfrom 10 to 100 μm, e.g. from 20 to 85 μm.

Generally, the width of a pore is less than 100 μm. The width of a poreis typically from 0.5 to 50 μm. Preferably the width of a pore is from 1to 25 μm, e.g. from 4 to 20 μm.

It is speculated that different kinds of coatings (e.g. varying in spikelength or the presence or absence of clusters) may be suited to the mostefficient heat transfer to different fluids. The density of spikes andcavities/pores, and their sizes, determine the density of bubbleformation and the sites into which those bubbles move and grow. Thisaffects heat transfer properties. The presence of clusters providespores, and so heat exchange elements having clusters are best suited forheat transfer to fluids to which heat may be efficiently transferred bythe formulation and growth of larger bubbles. Thus the coating in theheat exchange element of the invention may be varied to provideoptimised heat transfer to a range of different fluids having differentheat transfer characteristics. The fluids may include varied speciessuch as organic refrigerants, water, liquid N₂ or CO₂ and so on. Forinstance, the organic refrigerants may include hydrofluoroolefins(HFOs), hydrofluorocarbons (HFCs), fluorocarbons (FCs) and hydrocarbons.Another exemplary fluid is ammonia.

The thickness of the coating in the heat exchange element is notparticularly limited. The thickness of the coating may be defined as thelargest perpendicular distance from the substrate to the edge of thecoating material. Typically the thickness of the coating is at least 1μm, e.g. at least 2 μm. Generally the thickness of the coating is 200 μmor less. Preferably, the thickness of the coating is from 1 μm to 100μm, e.g. from 2 to 50 μm. In one embodiment, the invention provides aheat exchange element wherein the thickness of the coating is 10 μm ormore. In another embodiment, the invention provides a heat exchangeelement wherein the thickness of the coating is from 2 to 50 μm.

The exact weight of the coating per unit area of the substrate willdepend on the structure of the coating and the materials therein.Generally, the weight of the coating per unit area of substrate is atleast 10 g m². Generally, the weight of the coating per unit area ofsubstrate is no more than 900 g m². Usually, the weight of the coatingper unit area of substrate is from 20 to 500 g m⁻², preferably from 30to 400 g m⁻².

The coating comprises one or more metals. Generally, the coatingcomprises one or more transition metals. Preferably, the coatingcomprises one or more of vanadium, chromium, manganese, cobalt, nickel,and copper. In a preferred embodiment, the coating comprises copper,nickel or an alloy of copper and nickel. In a particularly preferredembodiment of the invention, the coating comprises copper. In anotherparticularly preferred embodiment, the coating comprises an alloy ofcopper and nickel.

The coating usually has a high metal content, i.e. it is primarilymetallic. Usually, the coating contains at least 50% metal by weight ofthe coating. In a preferred embodiment, the coating contains at least70% metal by weight of the coating. In one embodiment of the invention,the coating comprises 80% metal by weight of the coating. In aparticularly preferred embodiment, the coating has a very high metalcontent e.g. at least 90% or at least 99% metal by weight of thecoating.

The coating of the heat exchange element of the invention having astructure and properties as described above may conveniently be obtainedby electroless deposition. Thus, in one embodiment of the invention, thecoating is obtainable by electroless deposition. In an aspect of thisembodiment, the coating is obtained by electroless deposition. Forexample, the coating is typically obtained or obtainable by anelectroless flow deposition process as defined herein.

In one embodiment of the invention, the coating comprises one or more,e.g. one or two, surface layers on the coating. By “surface layer” ismeant a layer of material on the surface of the coating. A surface layeris therefore a layer of material on the side of the coating opposite tothe side of the coating which is in contact with the substrate.

The material of a surface layer is not particularly limited. Forexample, a surface layer may comprise one or more metal(s) or one ormore polymer(s). A surface layer may comprise one or more hydrophobicmaterial(s) and/or one or more hydrophilic material(s) to adjust thewettability of the heat exchange element. A surface layer may compriseone or more protective material(s) to protect the coating from wear andtear or the influence of harsh refrigerants such as ammonia.

Preferably a surface layer comprises one or more transition metal(s),particularly nickel or titanium. In a preferred embodiment, a surfacelayer consists of nickel, titanium, or an alloy comprising nickel and/ortitanium. Preferably a surface layer is a nickel layer. Typically thesurface layer comprising a transition metal is on an exposed surface ofthe heat exchange element, i.e. it comes into direct contact with thefluid flowing along the flow path. Typically, a single surface layer ispresent, preferably a single layer comprising a transition metal asdiscussed above.

The surface layer(s) are thin layer(s) in order to preserve theadvantageous structure of the coating. Generally the total thickness ofany surface layer(s) present is 500 nm or less. For example, the totalthickness of the surface layer(s) present may be from 1 to 250 nm, e.g.from 10 to 200 nm.

Substrate

The substrate is a solid object. The substrate generally takes the formof a typical heat exchange element or a part thereof or a heat exchangeror part thereof which is then coated according to the invention.

Examples of suitable substrates include: shell and tube heat exchangers,plate heat exchangers, brazed plate heat exchangers, gasketed heatexchangers, plate and shell heat exchangers, adiabatic wheel heatexchangers, plate fin heat exchangers, pillow plate heat exchangers,fluid heat exchangers, dynamic scraped surface heat exchangers, miniheat exchangers and microchannel heat exchangers. Other examples ofsuitable substrates include parts of heat exchangers such as a fin,plate, coil or tube that is part of a heat exchanger. Still otherexamples of suitable substrates include heat exchangers or parts of heatexchangers suitable for incorporation in a boiler, air conditioner,refrigerator, radiator, heat sink, solar collector or other type ofthermal transfer component.

The substrate is preferably a conductor of heat. The substrate maytherefore comprise metal. In one embodiment, the substrate is a metalobject including a metallic or metal alloy. For example, the substratemay be an object such as a heat exchange element made of one or more ofcarbon steel, austenitic stainless steel, martensitic steels, aluminiumand its alloys such as aluminium bronzes, aluminium silicon etc., copperand its alloys, titanium and zirconium. Preferably the substrate is anobject comprising stainless steel or titanium, or the substrate consistsof stainless steel or titanium. These metals are preferred as theyresist corrosion.

The substrate may be non-metallic, and comprise a semiconductor such assilicon, or gallium nitride. It may, for instance, comprise a carboncomposite that has a high thermal conductivity. In one embodiment, thesubstrate may be made of a carbon composite.

The substrate may comprise one or more outer layer(s). In the heatexchange element (e.g. the heat exchange element of the invention),where an outer layer is present, all or part of an outer layer of thesubstrate is located between the body of the substrate and the coating.In one embodiment, a single outer layer is present which is in contactwith the body of the substrate and the coating. In other embodiments,two or more outer layers are present. Typically if an outer layer ispresent, a single outer layer is present. More preferably, there is noouter layer such that the substrate is in direct contact with thecoating.

An outer layer, when used, typically comprises one or more metal(s) ormetal alloy(s). For instance, an outer layer may be a metallic layer. Anouter layer is useful in improving corrosion resistance of thesubstrate, particularly where the outer layer comprises titanium, nickelor stainless steel. An outer layer may also improve formation of thecoating during the manufacture of the heat exchange element of theinvention, and may improve adhesion of the coating to the substrate inthe heat exchange element.

The heat exchange element is suitable for transferring heat from itssurface to a fluid in contact with its surface. As discussed above, thestructure of the coating promotes efficient heat transfer by nucleateboiling and/or film boiling of a liquid, and so the heat exchangeelement of the invention is particularly suitable for transfer of heatto a liquid. Often, therefore, the substrate is an object suitable foror adapted for transferring heat to a liquid. In some embodiments, thesubstrate is a heat exchange element, heat exchanger or part of a heatexchanger that is designed for the transfer of heat to a liquid. In oneembodiment, the substrate is a heat exchanger suitable for transferringheat to a liquid.

In a particular embodiment of the invention, the heat exchange elementmay be suitable for fluid to fluid heat transfer, for example gas toliquid heat transfer or liquid to liquid heat transfer. Thus, thesubstrate may be a heat exchanger or part of a heat exchanger designedfor fluid to fluid heat transfer, for example gas to liquid heattransfer or liquid to liquid heat transfer.

A heat exchange element suitable for transfer of heat to a fluid such asa liquid has a surface or surfaces suitable for contacting a fluid. Afluid to which heat is transferred by a heat exchange element may bereferred to as a “working fluid” or “refrigerant”. Typically but notessentially, a heat exchange element suitable for transfer of heat to afluid such as a liquid comprises one or more flow channel(s) suitablefor carrying a fluid to which heat is to be transferred. Typically butnot essentially, therefore, the substrate comprises one or more flowchannel(s) suitable for carrying a fluid such as a liquid to which heatis to be transferred.

A fluid from which heat is transferred to a heat exchange element may bereferred to as a “heat transfer fluid” or “heating fluid”. A heatexchange element that is suitable for receiving heat from a fluid suchas a liquid typically comprises one or more flow channel(s) suitable fortransfer of heat from a fluid to the heat exchange element. Thus,typically but not essentially, the substrate comprises one or more flowchannel(s) suitable for carrying a liquid from which heat may betransferred to the substrate.

In one aspect of the invention, the heat exchange element (e.g. the heatexchange element of the invention) is a fluid to fluid heat exchanger,preferably a fluid to liquid heat exchanger such as a gas to liquid orliquid to liquid heat exchanger, or part thereof. Typically, therefore,the substrate comprises one or more flow channel(s) suitable forcarrying a fluid (e.g. a liquid) from which heat may be transferred tothe substrate, and one or more flow channel(s) suitable for carrying afluid (preferably a liquid) to which heat may be transferred from thesubstrate.

By “flow channel” is meant a channel along which fluid can pass throughthe substrate. A flow channel comprises one or more openings via whichfluid may enter and/or leave the flow channel. Such an opening may bereferred to as an inlet. In most configurations, a flow channel isenclosed on all sides along its length (i.e. a tube) and comprisesopenings at one or more ends. However, as the skilled person willappreciate, other configurations of a flow channel are possible.

In one embodiment of the heat exchange element, the flow path comprisesa flow channel and the coating is present on at least a part of thesurface of said flow channel. Typically the coating substantially coversthe surface of the flow channel. By the surface of the flow channel ismeant the internal surface of the flow channel. The internal surface ofa flow channel will come into contact with fluid flowing through theflow channel. The flow path may sit entirely within the flow channel.That is, the flow channel may extend beyond the flow path.Alternatively, the flow path may extend to or even beyond one or moreopenings (inlets) to the flow channel.

Since the coating is particularly useful for promoting heat transfer toa refrigerant (e.g. a liquid), the coating is advantageously present onthe surface or surfaces (such as the surface of a flow channel) that arefrigerant will contact when the element is in use. In order to avoidwastage of material, the coating may be present only on the surface orsurfaces of the heat exchange element which may contact a refrigerant.

The heat exchange element may comprise one or more flow channel(s) nothaving coating present on their surface. Such uncoated flow channels maybe useful for carrying a heat transfer fluid when the heat exchangeelement is in use. Such flow channels for heat transfer fluid are termed“first flow channels” herein. Typically, the heat exchange element isarranged such that the heat transfer fluid can transfer heat to therefrigerant, via the heat exchange element.

Where the heat exchange element of the invention comprises a flowchannel having coating present on the surface of the flow channel, thefirst region (having an average spike length S₁ therein) and the secondregion (having an average spike length S₂ therein) may both be locatedwithin the flow channel. Alternatively, one region may be located in theflow channel and the other may not. In one embodiment, the first regionis located at or near to an inlet to the flow channel and the secondregion is located at a greater distance from the inlet than the firstregion. For instance, where the first and second regions are within aflow channel, the coating may comprise longer spikes at or near theinlet and shorter spikes further along the flow channel, e.g the spikelength may gradually decrease from at or near the inlet to a pointfurther along the channel. A heat exchange element according to thisembodiment may be conveniently produced by the process of the invention.

In a preferred embodiment of the invention, the heat exchange element issuitable for transferring heat to a refrigerant, preferably to a liquidrefrigerant. When the heat exchange element is in use, it may compriseone ore more refrigerant(s); for instance, a refrigerant(s) may bepresent in one or more flow channels in the heat exchange element. Thus,in one embodiment, the invention provides a heat exchange element thatcontains a refrigerant. That is, the invention provides a working heatexchange element wherein the working heat exchange element comprises aheat exchange element of the invention and a refrigerant. In a furtheraspect of this embodiment, the heat exchange element also comprises aheat transfer fluid (for instance in one or more flow channels of theheat exchange element).

The refrigerant is a fluid, preferably a liquid, suitable for receivingheat. A wide variety of liquids are suitable including CO₂, nitrogen,ammonia, water, aqueous solutions, organic liquids including halogenatedalkanes such as CFCs, and sulphur-based refrigerants. The heat transferfluid is a fluid, usually a liquid capable of providing heat to the heatexchange element. A wide variety of liquids are suitable including waterand organic liquids such as oils.

Heat Transfer Efficiency of the Heat Exchange Element of the Invention

The heat exchange element of the invention is capable of highlyefficient heat transfer. The heat transfer efficiency of the heatexchange element of the invention is similar or better than the heattransfer efficiency of a comparable substrate having a sintered surface.However, advantageously, much less metal is needed to create the coatedheat exchange element of the present invention than is needed to preparea comparable substrate having a sintered surface.

The heat exchange element of the invention facilitates more efficientheat transfer than a comparable substrate having a polished surface. Inone embodiment, the heat exchange element of the invention has a heattransfer coefficient at least 20% higher than a comparable substratehaving a polished surface, for instance at least 30% higher or 50%higher than a comparable substrate having a polished surface. The heattransfer coefficients are typically calculated for the same system (i.e.having the same heat source and the same refrigerant and at the sametemperature or temperatures) in order to make the comparison. The heattransfer coefficients for the purpose of this comparison are typicallycalculated in a flow boiling regime and at a heat flux of less than 200kW m⁻², e.g. at 100 kW m⁻². A polished surface is a surface polishedwith emery paper of grade 1200. A polished surface typically has anaverage roughness of 0.04 μm or less, measured on a Taylor Hobsonsurface profiler (Taylor Surf Series 2, using Taylor Hobson ultrasoftware).

In one embodiment, the heat exchange element of the invention has a heattransfer of 7000 W m⁻² K⁻¹ or more at a heat flux of approximately 80 kWm⁻².

In one embodiment, the heat exchange element of the invention displays asuperheat that is 50% or less of that displayed by a comparablesubstrate having a polished surface, under the same test conditions.Preferably, the heat exchange element of the invention displays asuperheat that is 30% or less of that displayed by a comparablesubstrate having a polished surface, under the same test conditions.Typical test conditions include a pool boiling experimental regime, anda heat flux of up to 500 kW m⁻², e.g. 20 kW m⁻². The superheat is thedifference (in Kelvin) between the temperature of the surface from whichheat is being transferred to a fluid and the temperature of the fluid.

In one embodiment, at a heat flux of up to 500 kW m⁻² the heat exchangeelement of the invention displays a superheat of 10 K or less. In anaspect of this embodiment, at a heat flux of up to 200 kW m⁻² the heatexchange element of the invention displays a superheat of less than 10K. Typical test conditions include a pool boiling experimental regime.

Method of Heat Transfer

The heat exchange element of the invention is capable of facilitatingefficient heat transfer to its surroundings. In particular, the heatexchange element is capable of facilitating efficient heat transfer to afluid in contact with the element, and particularly to a fluid incontact with the coating of the element. The heat exchange elementdefines a flow path along which fluid may contact the element and whichis coated with the coating, along all or part of its length. Thus, theinvention provides a method of transferring heat to or from a fluidwhich comprises providing the fluid to a flow path of a heat exchangeelement of the invention.

As discussed above, the coating is adapted to promote heat transfer byfacilitating boiling of a liquid (in a nucleate and a film boilingregime). Accordingly, in preferred embodiments of the method of theinvention, the method comprises transferring heat to a liquid. In theseembodiments, the method comprises providing a liquid to a flow path of aheat exchange element of the invention. In one embodiment, the method isa method of transferring heat from a solid to a liquid. In anotherembodiment, which is preferred, the method is a method of transferringheat from a fluid to a liquid, e.g. from a gas to a liquid or a liquidto a liquid. In a particular embodiment, the method is a method oftransferring heat from a liquid to a liquid.

In some embodiments, the method of heat transfer comprises passing afluid (preferably a liquid) along a flow path of a heat exchange elementof the invention.

Generally, the method comprises transferring heat to a refrigerant, themethod comprising passing a refrigerant along the flow path of a heatexchange element of the invention. For example, the method may comprisepassing a refrigerant through a first flow channel (which defines theflow path) of the heat exchange element of the invention. In someembodiments, the method comprises transferring heat from a heat transferfluid by passing a heat transfer fluid along a second flow channel ofthe heat exchange element of the invention. In a preferred embodiment,the method is a method of transferring heat from a heat transfer fluidto a refrigerant, the method comprising passing said heat transfer fluidthrough a second flow channel of the heat exchange element and passingsaid refrigerant through a first flow channel of the heat exchangeelement. The flow channels are typically arranged to conveniently enableheat transfer from the heat transfer fluid via the heat exchange elementto the refrigerant.

The temperature at which the method of the invention is performed istypically less than 500° C. The temperature at which the method of theinvention is performed will depend on the refrigerant (if used).Typically, where a refrigerant is used, the method is performed at atemperature within 20° C. of the boiling temperature of the refrigerant,e.g. from 0 to 10° C. above the boiling temperature of the refrigerant.

Also provided by the invention is the use of a heat exchange elementaccording to the invention as a heat exchanger. In one embodiment, theinvention provides the use of a heat exchange element in a method ofheat exchange as described herein.

Process for Producing Heat Exchange Element

The coating of the heat exchange element of the invention mayconveniently be created by electroless deposition. Thus, the inventionprovides a process for producing a heat exchange element according tothe invention, the process comprising providing an electrolessdeposition solution to a surface of a substrate. In one aspect, theelectroless deposition process is a bath process. In another aspect, theelectroless deposition process is a flow process. These specific aspectswill be described in further detail in later sections; the followingcomments concerning electroless deposition processes apply equally tobath processes and flow processes. The heat exchange element of theinvention and substrate are as defined herein.

The electroless flow deposition process of the invention, however, canprovide heat exchange elements which are not heat exchange elements ofthe invention. That is, the electroless flow deposition process canprovide coatings which differ from that of the heat exchange element ofthe invention. The invention therefore provides an electroless flowdeposition process for producing a heat exchange element, and heatexchange elements obtained or obtainable by that process. In a preferredaspect, the electroless deposition process of the invention is a processfor producing a heat exchange element of the invention. The heatexchange element and substrate are as defined herein.

Electroless deposition involves the reduction of metal ions in solutionto produce metal atoms deposited on the substrate surface to form acoating comprising metal, as described above. The electroless platingprocess is a non-electrolytic process. The electroless plating processdoes not require molten metal, unlike hot dip galvanising processes.

Advantageously, electroless deposition may be performed at lowtemperatures. Typically, the electroless deposition process is performedat a temperature of from 20° C. to 120° C. In one embodiment, theelectroless deposition process is performed at room temperature.Preferably, the electroless deposition process is performed at atemperature of 100° C. or less, for example from 20° C. to 100° C. or50° C. to 100° C., e.g. at approximately 60° C., 70° C. or 80° C. Itshould be noted that performing electroless deposition within thistemperature range means that the electroless deposition solution ismaintained within the aforementioned temperature range, typically 20° C.to 120° C., during electroless deposition. Preceding and subsequentmethod steps may be performed at temperatures within or outside theabove-mentioned range.

The structure of the coating produced by the electroless depositionprocess is influenced by the electroless deposition conditions at thesurface of the substrate. In particular, varying the deposition time,the concentration of ions in the electroless deposition solution and thetemperature will influence the structure of the coating formed by theelectroless deposition process.

The electroless deposition solution comprises one or more metal ions.The metal ion(s) are deposited onto the surface of the substrate to forma coating comprising metal during the electroless deposition process.The metal ion(s) are generally selected from one or more of a vanadium,chromium, manganese, cobalt, nickel, or copper ion. Preferably, theelectroless deposition comprises copper and/or nickel ions. Particularlypreferably, the electroless deposition solution comprises copper ions.For example, the electroless deposition solution may comprise one ormore of Cu²⁺, Cu⁺ and Ni²⁺.

The electroless deposition solution typically comprises a reducingagent. The choice of reducing agent will depend on the one or more metalions in the solution and on the nature of the substrate. Suitablereducing agents include, for example, one or more of iodate;oxyphosphorus ions such as phosphate, phosphite and hypophosphite; orborate ions.

The electroless deposition solution is typically an aqueous solution.However, the electroless deposition solution may comprise solvents otherthan water such as alcohols or ethers. Usually, the primary solvent inthe electroless deposition solution is water. The electroless depositionsolution may also comprise one or more complexing agent(s) and/or one ormore stabiliser(s) and/or one or more modifier(s), the choice of whichdepends on the substrate and the materials to be deposited.

The structure of the coating formed by the electroless depositionprocess is influenced by the concentration of metal ions and reductantat the surface of the substrate. A higher concentration of metal ionstends to increase the amount of material deposited by electrolessdeposition. (By “a higher concentration of metal ions” is meant a higherconcentration of the metal ions that are deposited onto the surfaceduring the electroless deposition process). A higher concentration ofmetal ions also tends to increase the thickness of the coating formed. Ahigher concentration of metal ions also favours the formation of largersurface features. For example, a higher concentration favours theformation of longer spikes and/or a greater density of spikes on thesurface. Thus, the formation of a coating according to the invention(comprising a first region wherein the average spike length is longerthan that in a second region) may be achieved by providing variation inconcentration of the electroless deposition solution over the surface ofthe substrate. Thus, in some embodiments, the invention provides anelectroless deposition process which comprises:

-   -   providing an electroless deposition solution having a        concentration C₁ to a first region of the substrate;    -   providing an electroless deposition solution having a        concentration C₂ to a second region of the substrate;    -   wherein C₁ is greater than C₂.

C₁ and C₂ are the concentrations of metal ions which may be deposited onthe substrate in the electroless deposition solution or solutions.

The structure of the coating formed by the electroless depositionprocess is also influenced by the time for which the electrolessdeposition is allowed to occur. Generally, the longer the time allowedfor electroless deposition, the thicker the coating will be. Similarly,a longer deposition time favours the formation of larger surfacefeatures. For instance, a longer deposition time favours the formationof longer spikes and/or a greater density of spikes on the surface.Usually, the process comprises providing the electroless depositionsolution to a surface of a substrate for at least 15 minutes. Alsousually, the process comprises providing the electroless depositionsolution to a surface of a substrate for period of 12 hours or less. Inone embodiment, the process of the invention comprises providing theelectroless deposition solution to a surface of a substrate for a periodof from 0.5 hours to 10 hours. For example, the period may be from 1hour to 5 hours. Evidently, the electroless deposition process can coata heat exchange element in an advantageously short time.

In the context of an electroless flow deposition process, reference to“providing the electroless deposition to a surface” should be taken asmeaning “flowing the electroless deposition solution over a surface”.That is, “providing” should be taken to mean “flowing” or “flowingover”. For example, where the process is a flow process, the processusually comprises flowing the electroless deposition solution over asurface of a substrate for at least 15 minutes. Also usually, theprocess comprises flowing the electroless deposition solution over asurface of a substrate for period of 12 hours or less. In oneembodiment, the process of the invention comprises flowing theelectroless deposition solution over a surface of a substrate for aperiod of from 0.5 hours to 10 hours. For example, the period may befrom 1 hour to 5 hours.

In some embodiments, the electroless deposition solution is notrefreshed during the electroless deposition process. If the electrolessdeposition solution is not refreshed, the electroless depositionsolution will become depleted over time during the process of theinvention. By “depleted” is meant that the concentration of metal ionsand reductants in the solution have fallen below their initial value(the value at the start of the process). The electroless depositionsolution may be said to be depleted by 5% once the concentration in thesolution of metal ions to be deposited has fallen by at least 5%, to 95%or less of its original value. That is, if the electroless depositionsolution is suitable for depositing copper ions, the solution is said tobe depleted by at least 5% once the concentration of copper ions insolution has fallen to 95% or less of its original value.

It should be noted that localised variations in concentration may occur.Depletion is therefore considered in terms of the depletion of theelectroless deposition solution taken as a whole. For instance, multiplesamples of the electroless deposition solution may be taken in order toindicate the depletion in the solution taken as a whole. For instance,in a bath process, multiple samples may be taken from the bath while theelectroless deposition solution is agitated. In another example, wherethe process comprises flowing electroless deposition solution from areservoir over the surface and back into the reservoir, one or moresamples may be taken from the reservoir. Similarly, one or more samplesmay be taken from the solution flowing from the reservoir to thesubstrate. Alternatively or additionally, one or more samples may betaken from the solution flowing from the substrate back into thereservoir.

Once the electroless deposition solution is considerably depleted (forinstance by 50% or more), the concentration of ions in solution may below and hence the deposition process will become undesirably slow.Accordingly, the electroless deposition process of the invention isusually continued until the electroless deposition solution is depletedby 50% or more.

In one embodiment, the process for producing a heat exchange elementcomprises providing an electroless deposition solution to a surface ofthe substrate for a time T, wherein T is the time taken for theelectroless deposition solution to become depleted by 5 to 50%. That is,until the concentration of metal ion to be deposited has fallen to 50%to 95% of its original value. Preferably, T is the time taken for theelectroless deposition solution to become depleted by 10 to 40%, or byno more than 30%. For instance, the process for producing a heatexchange element (e.g. a heat exchange element of the invention) maycomprise flowing an electroless deposition solution over a surface ofthe substrate for a time T.

The extent of depletion may be determined by, for example, noting thechange over time in ion concentration or conductivity of the electrolessdeposition solution. Thus, in one embodiment, the process comprisesmeasuring the ion concentration of the solution. In another embodiment,the process comprises measuring the conductivity of the solution.“Measuring” may comprise monitoring the change in a particular valueover the course of the electroless deposition process.

The skilled person would appreciate that a wide variety of methods aresuitable for measuring the ion concentration of the electrolessdeposition solution, and hence for determining the extent of depletion.These methods may be performed on one or more sample(s) taken from theelectroless deposition solution. Alternatively they may be performedin-line, e.g. within a bath or reservoir of electroless depositionsolution that is used for electroless deposition. Suitable methodsinclude optical methods such as colorimetry. Colorimetric methods areparticularly well suited for solutions of Cu²⁺ ions which are highlycoloured. Other methods include anodic stripping voltammetry, ionchromatography and ion emission spectroscopy (e.g. inductively coupledplasma optical emission spectroscopy, ICP-OES). Thus in some embodimentsthe process comprises measuring the ion concentration in the electrolessdeposition solution, for example by any of the above methods. In thecontext of this measurement, “ion concentration” includes theconcentration of metal ions which are deposited on the substrate duringelectroless deposition.

Where the process comprises flowing electroless deposition solution overa substrate, ICP-OES may be used to analyse the electroless depositionsolution flowing over the substrate both before and after it contactsthe substrate. This can reveal the maximum and minimum ionconcentrations to which the substrate is exposed. The ion concentrationof the solution after it has contacted the substrate may be used todetermine whether the solution flowing from the substrate back into areservoir needs to be dosed with additional ions to adjust itsconcentration. In the flow process of the invention, the ionconcentration in the reservoir may be measured periodically to determinewhether the reservoir itself needs to be dosed with additional ions toadjust its concentration. Similarly, the ion concentration of a bathdetermined during a bath process may be used to determine if dosing isrequired to increase ion concentration.

The desired spike length in the coating of the heat exchange element ofthe invention may be achieved by adjusting the initial ion compositionof the electroless deposition solution, and/or the length of time forwhich the deposition process is allowed to occur.

The amount of time for which electroless deposition is allowed to occuraffects not only the spike length in the coating formed by the process,but also the incidence of clusters of spikes in the coating. The longerthe time for which electroless deposition is allowed to occur, thegreater the likelihood that clusters of spikes will form. Thus, theprocess for producing a heat exchange element may be adjusted to formclusters by increasing the amount of time for which electrolessdeposition is allowed to occur.

The exact amount of time needed to form clusters will vary with thesubstrate, the composition of the electroless deposition solution, andthe temperature.

The process may also be adapted to promote the formation of clusters byrepeating the process more than once. A process for producing a heatexchange element of the invention having a coating comprising clustersmay therefore comprise:

-   -   (i) performing the electroless deposition process of the        invention as described herein;    -   (ii) activating the surface of the coating of the heat exchange        element thus produced, for instance by immersing the heat        exchange element in an a solution of PdCl₂ to form an activated        heat exchange element; and    -   (iii) repeating the electroless deposition process of the        invention.

Bath Process

In one aspect, the process of the invention is a bath process (forproducing a heat exchange element of the invention). In a bath process,the electroless deposition solution is provided to the surface of asubstrate by placing the substrate in a bath of electroless depositionsolution. In some embodiments, the electroless deposition solution isagitated during the electroless deposition process. Agitation of theelectroless deposition solution reduces variation in the composition ofthe electroless deposition solution (such as variation in local metalion concentration) throughout the bath.

In some embodiments, the entire surface or surfaces of the substrate arecoated during the bath process. In other embodiments, part of thesurface or surfaces of the substrate are protected before the substrateis placed into the bath such that the protected parts are not coated. Instill further embodiments, part of the surface or surfaces of thesubstrate are not capable of adhering to the coating and hence are notcoated during immersion in the bath.

In some embodiments, multiple heat exchange elements according to theinvention may be produced simultaneously by placing two or moresubstrates into a bath.

In one aspect, the bath process may be used to provide a coating havinga first region wherein the spike length is S₁ and a second regionwherein the spike length is S₂ by adjusting the length of time of theelectroless deposition process. For example, the substrate may be dippedinto the bath slowly such that the part or parts of the substratesurface which enter the bath first experience a longer exposure to theelectroless deposition solution than part or parts of the substratesurface which enter the bath subsequently. The coated part or parts ofthe heat exchange element thus produced which spent the longest timeexposed to the electroless deposition solution will typically have thelongest average spike length. If the substrate is dipped into the bathat a constant slow speed, the coating produced on the substrate may showa smooth variation in average spike length. In another example, aprotective covering may be removed from a portion of the substrate partway through the electroless deposition process, leaving that portion ofthe substrate to be coated during the remaining immersion time.

In another aspect, the bath process may be used to provide a coatinghaving a first region wherein the spike length is S₁ and a second regionwherein the spike length is S₂ by allowing the concentration of theelectroless deposition solution to vary across the surface. Where asubstrate comprises a channel into which the electroless depositionsolution can flow, solution will usually flow into said channel when thesubstrate is dipped into the bath of solution. As the solution flowsinto the channel, electroless deposition occurs and the concentration ofmetal ions to be deposited in the solution flowing through the channelis decreased. Fresh solution flows into the channel from the bath overtime, but always has the greatest concentration of metal ions to bedeposited (corresponding approximately to the higher metal ionconcentration in the surrounding bath) at the entrance to the flowchannel. Thus a concentration gradient is formed, a higher concentrationof metal ions to be deposited being found at the entrance or entrancesto the flow channel and a lower concentration of metal ions to bedeposited occurring further into the flow channel. This effect isparticularly pronounced where the flow channel is long and/or narrow.The longest spikes form where the concentration of metal ions to bedeposited is highest; shorter spikes form elsewhere.

Flow Process

In one aspect, the process for producing a heat exchange elementcomprises flowing an electroless deposition solution over a surface of asubstrate. In some embodiments, this electroless flow deposition processproduces a heat exchange element according to the invention.

Flowing an electroless deposition solution over a surface of a substratecomprises providing a flow of electroless deposition solution movingover a surface of the substrate at a non-zero flow rate. The flow ofsolution is understood to be “over” a surface of the substrate if it isin contact with said surface of the substrate. The flow may be providedover one or more surfaces of the substrate, for example external and/orinternal surfaces of the substrate, an internal surface being, forexample, the surface of a channel (e.g. a tube) passing through thesubstrate. The flow of solution may be provided over the entiresubstrate or over only part of the substrate.

In an electroless deposition process that occurs in a static environmentsuch as a bath, there is usually no net direction of flow of electrolessdeposition solution over any point on the substrate surface. Thesolution is typically agitated to provide movement of solution withinthe bath, but that movement typically has no net direction over thecourse of the static electroless deposition process (e.g. bathelectroless deposition process). In a static environment the directionof flow of electroless deposition solution over any point on thesubstrate surface may change frequently and at random during the courseof the electroless deposition process.

The flow electroless deposition process provides a net direction of flowof electroless deposition solution over any point on the substratesurface subjected to the flow. The direction of flow of electrolessdeposition solution over any point on the substrate surface subjected tothe flow typically does not change during the deposition process. Thedirection of flow is usually constant during electroless flowdeposition. However, the direction of flow provided to the surface ofthe substrate may be deliberately changed during the course of the flowprocess.

As the skilled person will appreciate, the precise details of theprocess used to provide a flow of electroless deposition solution over asubstrate (for example the apparatus) may vary. In a particularly simplesetup, the process may comprise pouring the electroless depositionsolution from a container over a surface of a substrate. More usually,the process will comprise generating flow of electroless depositionsolution using a flow generator (e.g. a pump), and providing the flow ofelectroless deposition solution to a surface of the substrate via aconduit.

In the flow process of the invention, the electroless depositionsolution flows over the surface of a substrate. The path taken by theflow of solution over the substrate surface is referred to as “thesolution flow path”. In some embodiments, the coating is formed over thewhole of the solution flow path. In other embodiments, such asembodiments wherein part of the substrate on the solution flow path ismasked to prevent electroless deposition, the coating is formed overpart of the solution flow path. Preferably, the coating is formed overthe whole of the solution flow path.

The flow of electroless deposition solution over a surface of asubstrate creates a concentration gradient over the said surface of thesubstrate. The electroless deposition solution provided to the surfacecontains metal ions to be deposited. The concentration of such metalions in the electroless deposition solution before it is provided to thesurface of the substrate may be referred to as C₁. Once the electrolessdeposition solution contacts the substrate (that is, once the solutioncontacts a part of the surface of the substrate which is susceptible tobeing coated by electroless deposition) electroless deposition willoccur. This causes metal ions to come out of solution and reduces theconcentration of metal ions to a value lower than C₁. Thus as theelectroless deposition solution flows over the substrate, it isdepleted. A concentration gradient forms. The concentration gradientlies along the solution flow path. The concentration of metal ions inthe solution decreases as the distance the solution has travelled overthe substrate surface increases.

As discussed above, the concentration of metal ions tends to affect thethickness of the coating and the size of surface features formed by theprocess of the invention. Thus the process of the invention may providea coating that varies in thickness, and/or that shows varying size ofsurface features, along the path taken by the flow of solution over thesubstrate surface. A surface feature is a structure in the coating suchas a spike. In one embodiment, the thickness of the coating decreasesalong the solution flow path. The coating may be thickest at or near anend of the solution flow path, usually the end at which the electrolessdeposition solution first contacts the surface of the substrate duringthe process of the invention. In another embodiment, the size of thesurface features decreases along the solution flow path. The surfacefeatures may be largest at or near an end of the solution flow path,usually the end at which the electroless deposition solution firstcontacts the surface of the substrate during the process of theinvention.

The formation of longer spikes is favoured by a higher concentration ofmetal ions in the electroless deposition solution. Thus, the electrolessflow deposition process can produce a coated heat exchange element, thecoating comprising a first region wherein the average spike length is S₁and a second region wherein the average spike length is S₂. Theelectroless deposition flows along a route across the surface orsurfaces of the substrate, that route being the flow path of theelectroless deposition solution. Typically, the first region is at ornear the end of that flow path, and the second region is also on theflow path.

In some embodiments, the process comprises flowing an electrolessdeposition solution over a surface of the substrate along a single flowpath in a single direction. In other embodiments, the flow processcomprises a step of changing the flow direction. The flow process mayadditionally or alternatively comprise a step of changing the flow pathof the electroless deposition solution over the surface or surfaces ofthe substrate.

Where the process comprises changing (e.g. reversing) the direction offlow or the flow path of the electroless deposition solution, theprocess comprises altering the direction of the concentration gradientover the surface of the substrate. Thus, the position or region on thesurface of the substrate at which the thickest coating or largestfeatures (e.g. the longest spikes) form will move to the new position,corresponding to the new position of the substrate surface which is incontact with the electroless deposition solution having a maximumconcentration, C₁. This may cause the creation of a further region, athird region, having an average spike length different to or the same asthat in the first or second regions.

For example, where the flow of electroless deposition solution isprovided along a flow channel, the flow channel may comprise a thickercoating and/or larger features at or near the end of the flow channelwhere the electroless deposition solution entered the channel, and athinner coating and/or smaller features further along the channel. Ifthe process comprises flowing an electroless deposition solution intoone end of a flow channel of a substrate (the inlet) and out of anotherend of the flow channel (the outlet), the thickest coating/largestfeatures will be located at or near the inlet and the thinnestcoating/smallest will be located at or near the outlet. However, if thedirection of flow is reversed during the electroless deposition process,the thinnest coating/smallest features will be located at or near themiddle of the channel and the thicker coating/largest features will belocated at or near each end of the channel.

For example, where the flow of electroless deposition solution isprovided along a flow channel, the first region may be located at ornear an end of the flow channel and the second region may be locatedfurther along the flow channel. Thus, the flow channel may compriselonger spikes at or near the end of the flow channel where theelectroless deposition solution entered the channel, and shorter spikesfurther along the channel. If the process comprises flowing anelectroless deposition solution into one end of a flow channel of asubstrate (the inlet) and out of another end of the flow channel (theoutlet), the longest spikes will be located at or near the inlet and theshortest spikes will be located at or near the outlet. However, if thedirection of flow is reversed during the electroless deposition process,the shortest spikes will be located at or near the middle of the channeland longer spikes will be located at or near each end of the channel.

The coating thickness and/or feature size, e.g. the spike length,produced in the electroless deposition process can be adjusted bycontrolling the flow rate. The faster the flow rate, the thinner thecoating and/or the smaller the surface features produced. For instance,the faster the flow rate, the smaller the spikes produced. Additionally,increasing the flow rate reduces the size of the concentration gradientand hence reduces the variation in coating thickness and/or feature size(e.g. spike length) along the flow path of the electroless depositionsolution.

In some embodiments, the process of the invention comprises continuallyproviding fresh solution to the substrate. In other embodiments, theelectroless deposition solution is recycled. In one such embodiment, theprocess comprises:

-   -   flowing the electroless deposition solution from a reservoir of        electroless deposition solution over the surface of the        substrate; and    -   returning the electroless deposition solution to the said        reservoir.

By “reservoir” is meant a volume of electroless deposition solution.Typically the composition of the electroless deposition solution in thereservoir is not adjusted by an external source. For example, thereservoir is typically not topped up from an external source ofelectroless deposition solution (during the electroless depositionprocess).

Generally, the electroless deposition solution is provided to thesurface of the substrate by pumping.

During electroless deposition, the process generally comprises flowingelectroless deposition solution over the substrate surface at a flowrate of at least 10 mL/min. Typically, the process comprises flowingelectroless deposition solution over the substrate surface at a flowrate of at least 50 mL/min or at least 100 mL/min, preferably at least 1L/min. The aforementioned flow rates are generally maintained for atleast one minute, for instance for at least ten minutes, e.g. for atleast 30 minutes or 1 hour.

The process may involve varying the flow rate of the electrolessdeposition solution. In one embodiment, the process comprises:

-   -   flowing an electroless deposition solution over a surface of the        substrate at a first flow rate F₁; and    -   flowing an electroless deposition solution over the said surface        of the substrate at a second flow rate F₂.

Fand F₂ are typically different. F₁ and F₂ are typically at least 10mL/min. For example, F₁ and F₂ may be at least 50 mL/min or 100 mL/min.In one embodiment, F₂ is greater than F₁. For example, F2 may be twiceas large as F₁. e.g. F₂ may be 2 to 50 times as large as F₁. In anotherembodiment, F2 is greater than F₁. For example, F₁ may be twice as largeas F₂. e.g. F₁ may be 2 to 50 times as large as F₂. This latterembodiment may be useful to ensure that the substrate is rapidly coveredwith electroless deposition solution, before the flow rate is adjustedto a suitable deposition flow rate.

In some embodiments, F₂ is sufficiently large to reduce the adherence ofhydrogen bubbles from the substrate during electroless deposition. F₂may be large enough to force hydrogen bubbles away from the substratesurface during electroless deposition. In some embodiments, both F₁ andF₂ are sufficiently large to reduce the adherence of hydrogen bubblesfrom the substrate during electroless deposition, and/or to forcehydrogen bubbles away from the substrate surface during electrolessdeposition. In some embodiments, F₁ and/or F₂ are sufficiently large toforce electroless deposition solution into a flow channel of thesubstrate.

In one aspect, the process comprises:

-   -   pumping an electroless deposition solution from a reservoir over        a surface of the substrate at a first flow rate F₁ and returning        the electroless deposition solution to the reservoir; and    -   pumping an electroless deposition solution from a reservoir over        the said surface of the substrate at a second flow rate F₂ and        returning the electroless deposition solution to the reservoir.

In some cases, the process comprises flowing an electroless depositionsolution over a surface of the substrate at a first flow rate F₁ for aninitiation period before changing the flow rate to F₂. For instance, thesolution may be provided at a flow rate F₁ until all the surfaces of thesubstrate which are to be coated are covered with the solution.

In some embodiments, the process comprises pumping the electrolessdeposition solution to cause the electroless deposition solution to flowover a surface of the substrate. For example, the process may comprisepumping solution from a reservoir to create a flow of electrolessdeposition solution over a surface of the substrate. Alternatively oradditionally the process may comprise pumping electroless depositionsolution away from the substrate, for instance out of one or more flowchannels in the substrate. A suitable pump is any kind of devicesuitable for creating a flow of a fluid, particularly of liquid.

In some embodiments, the substrate comprises one or more flowchannel(s), and the process the process comprises flowing an electrolessdeposition solution through said one or more of said flow channel(s).That is, the solution flow path comprises one or more of the said flowchannel(s). In some embodiments, the substrate comprises a flow channel,and the process the process comprises flowing an electroless depositionsolution through said flow channel.

By flow channel is meant a path by which fluid can pass through thesubstrate. In these embodiments the process comprises contacting theelectroless deposition solution with the surface of said flow channel,usually the inner surface of said flow channel.

In one aspect of this embodiment, the process may involve coating one ormore flow channel(s) that are suitable for and/or intended for carryingrefrigerant. In a preferred aspect of this embodiment the processcomprises flowing an electroless deposition liquid only over a surfaceor surfaces of the substrate which are for contacting a refrigerant. Forinstance the substrate may be suitable for use as a fluid-fluid heatexchanger having a region for carrying the refrigerant fluid (a fluid towhich heat is transferred from the heat exchange element) and a regionfor the heat transfer fluid (a fluid which transfers heat to the heatexchange element), and the process comprises flowing electrolessdeposition solution only over one or more surfaces which are suitablefor and intended for contacting a refrigerant (i.e. the refrigerantfluid). This embodiment is advantageous as it ensures that noelectroless deposition solution is wasted in coating surfaces notintended for heat transfer.

The electroless flow deposition process is advantageous as it enablesthe flow electroless deposition solution to be controlled such that onlythe surfaces of the substrate which are intended to be coated may comeinto contact with the solution. This reduces wastage of the solution.The flow of solution may be controlled by, for example, connecting areservoir of solution to the inlet or inlets of those flow channelswhich are intended to be coated. Other flow channels may be blocked.

Advantages of the Electroless Flow Deposition Process (Flow Process)

As explained above, the flow process of the invention reduces the effectof hydrogen bubbles on the coating of a substrate by electrolessdeposition. The flow process therefore provides a heat exchange elementwhich is robust, durable and resistant to corrosion.

The flow process has various other advantages. For example, the flowprocess can advantageously be used to coat substrates having smalldepressions (e.g. small holes or channels therein) which may not beconveniently coated by a bath electroless deposition process. Where asubstrate having small depressions is placed into a bath of liquid,pockets of air may be trapped in those depressions, preventing theelectroless deposition solution from contacting the substrate hiddenbeneath the trapped air. The flow process of the invention is generallyperformed at a sufficiently high flow rate to force such air pocketsaway from the substrate surface and thus to ensure that all parts of thesubstrate which are exposed to the electroless deposition solution arecoated. The flow process of the invention is therefore suitable forcoating, for instance, substrates comprising very narrow channels,particularly substrates suitable for use in electronics cooling.

Another advantage of the electroless flow deposition process is that itreduces wastage of deposition solution. In the flow process of theinvention, the flow path of the electroless deposition solution over thesubstrate surface may be controlled. Consequently, electrolessdeposition solution may be provided only to those parts of the surfacewhich are to be coated in the process of the invention. By contrast, ina bath process, the entire substrate is usually immersed in theelectroless deposition solution and any parts of the substrate surfacewhich are not intended to be coated are masked by a protective coating.This may result in electroless deposition occurring on the protectivecoating, wasting material.

A further advantage of the flow process of the invention is that variousparameters of the process may be controlled in order to adjust thesurface structure obtained. For example, the flow rate of theelectroless deposition solution over the surface, the temperature atwhich the process is performed, the composition of the electrolessdeposition solution and so on may all be altered to adjust the structureof the coating produced by the process. Particularly advantageousstructures for heat exchange are achieved where the electrolessdeposition solution comprises copper ions, due to the high conductivityof the resulting copper coating.

The process can create a wide array of coatings suitable for heattransfer in various conditions. For instance, it may be used to producea heat exchange element suitable for transferring heat to a fluid,preferably to a liquid. In one embodiment the process is for producing aheat exchange element with a coating suitable for use in an evaporativeheat exchanger (a heat exchanger which cools a fluid by heating anotherfluid to the point of evaporation). In another embodiment the process isfor producing a heat exchange element with a coating suitable fortransferring heat by boiling a liquid.

A particular advantage of the flow process of the invention is that itmay be used to retrofit an electrolessly deposited coating to anexisting heat exchanger in situ and without disassembly.

Aspects of Electroless Flow Deposition Process

The following specific aspects of the electroless flow depositionprocess are provided.

-   1. A process for producing a heat exchange element comprising a    substrate and a coating, wherein:    -   the coating comprises a metal; and    -   the process comprises flowing an electroless deposition solution        over a surface of the substrate.-   2. A process according to aspect 1 wherein the process is performed    at a temperature of from 20° C. to 120° C.-   3. A process according to aspect 1 or aspect 2 wherein the    electroless deposition solution is an aqueous solution.-   4. A process according to any preceding aspect wherein the    electroless deposition solution comprises copper and/or nickel ions.-   5. A process according to any preceding aspect wherein the process    comprises:    -   flowing the electroless deposition solution from a reservoir of        electroless deposition solution over the surface of the        substrate; and    -   returning the electroless deposition solution to the said        reservoir.-   6. A process according to any preceding aspect wherein the process    comprises flowing the electroless deposition solution over the    surface of the substrate for a period of from 0.5 hours to 10 hours.-   7. A process according to aspect 5 or aspect 6 wherein the process    comprises flowing the electroless deposition solution over the    surface of the substrate for a time T, wherein T is the time taken    for the electroless deposition solution to become depleted by 5 to    50%.-   8. A process according to any preceding aspect wherein the process    comprises monitoring the ion concentration in the electroless    deposition solution.-   9. A process according to any preceding aspect wherein the process    comprises:    -   flowing an electroless deposition solution over a surface of the        substrate at a first flow rate F₁; and    -   flowing an electroless deposition solution over the said surface        of the substrate at a second flow rate F₂.-   10. A process according to aspect 9 wherein F₂ is greater than F₁.-   11. A process according to any preceding aspect wherein the process    comprises pumping the electroless deposition solution to cause the    electroless deposition solution to flow over a surface of the    substrate.-   12. A process according to any preceding aspect wherein the    substrate comprises a flow channel, and the process the process    comprises flowing an electroless deposition solution through said    flow channel.-   13. A process according to any preceding aspect wherein the process    comprises:    -   (i) providing an acid to a surface of the substrate, and/or    -   (ii) activating a surface of the substrate,

wherein steps (i) and/or (ii) are carried out prior to flowing anelectroless deposition solution over the surface of the substrateaccording to any preceding aspect.

-   14. A process according to any preceding aspect wherein the process    comprises applying a surface layer, preferably a surface layer    comprising nickel, to a heat exchange element produced according to    any one of aspects 1 to 13.-   15. A heat exchange element obtainable or obtained by a process    according to any preceding aspect.

Additional Process Steps

Usually, prior to electroless deposition the surface of the substrate isprepared by exposure to acid and to an activation solution. Thus, in oneembodiment, the process for producing a heat exchange element comprises:

-   -   (i) providing an acid to a surface of the substrate; and/or    -   (ii) activating a surface of the substrate,

wherein steps (i) and/or (ii) are carried out prior to providing anelectroless deposition solution to the surface of the substrate.

The function of the acid is typically to clean and optionally also toetch the surface. Suitable acids include sulphuric acid, hydrochloricacid or nitric acid. Where the substrate is a steel substrate, the acidused is preferably sulphuric acid. Where the substrate is a coppersubstrate, the acid used is preferably hydrochloric acid. The acid usedis typically strong acid, for instance 20% acid or above. Generally, thestep of exposure to acid is performed at room temperature or above, forinstance from 20° C. to 120° C., usually from 50° C. to 100° C. Usuallythe substrate is exposed to acid for a minute or more, preferably from 1minute to 1 hour. The substrate is usually rinsed with water afterexposure to acid.

The step of activating the surface may involve providing ametal-containing solution to the surface, for instance an aqueoussolution comprising metal ions. An exemplary activating solution isPdCl₂ solution. Activation is generally performed at a temperature ofbetween 0° C. and 100° C., usually at room temperature. Typically thesubstrate is rinsed after the activation step and prior to electrolessdeposition.

Other steps which may be performed prior to the electroless depositionprocess include, for instance, applying a protective mask to a part ofthe substrate surface to prevent application of coating to that part ofthe substrate.

In some embodiments, the process for producing a heat exchange elementof the invention comprises one or more steps performed after theelectroless deposition process. In one embodiment, the process comprisesapplying a surface layer, preferably a surface layer comprising a metal,e.g. nickel or tin (preferably nickel), to a heat exchange elementproduced by a method as described herein.

Experimental Protocol

Exemplary methods of applying a coating to a substrate to produce a heatexchange element of the invention are described below.

-   1. Bath process    -   i. Any part of the substrate which is not intended to be coated        is protected, for instance by applying stop-off lacquer thereto.        This step may be unnecessary if it is intended to apply coating        to all parts of the substrate that will be placed in the bath.    -   ii. The substrate is pre-heated to a temperature of 80° C. by        submerging the substrate in a water bath at 80° C.    -   iii. The substrate is then transferred to a 20% sulphuric acid        bath at 80° C. and left for 15 minutes. The substrate is then        rinsed in deionised water.    -   iv. The substrate is placed in a PdCl₂ solution (1 g L⁻¹) for 2        minutes. The substrate is then rinsed in deionised water.    -   v. The substrate is then re-heated to 80° C. as in (ii).    -   vi. The substrate is then placed in a bath of nanoFLUX        electroless deposition solution at 75° C. for two hours. The        nanoFLUX solution comprises 0.01M to 0.1M CuSO₄, 0.001M to 0.01M        NiSO₄, 0.1M to 0.5M NaH₂PO₂, 0.001M to 0.1M Na₃C₆H₅O₇, 0.1M to        1M HBO₃, Janus Green 0-700 ppm, PVP 0-200 ppm, CTAB 0-300 ppm,        SBS 0-500 ppm and 0 to 200 ppm PEG. The solution in the bath is        agitated continuously during this time. The bath contains at        least 10 litres of solution per square metre of substrate        surface to be coated.    -   vii. The coated substrate is removed from the bath and rinsed in        deionised water.    -   viii. The coated substrate is dried in an oven.

The above experimental protocol can be modified to produce a coatingcomprising clusters by adjusting step (vi) such that:

-   -   vi. The substrate is then placed in a bath of nanoFLUX        electroless deposition solution at 75° C. for four hours. The        solution in the bath is agitated continuously during this time.        The bath contains at least 50 litres of solution per square        metre of substrate surface to be coated.

-   2. Flow process

This flow process protocol is for coating one square metre of asubstrate's surface.

-   -   i. Any part of the substrate which is not intended to be coated        is protected, for instance by applying stop-off lacquer thereto.        This step may be unnecessary if the intended flow path of the        electroless deposition solution contacts only the part or parts        of the substrate which are intended to be coated.    -   ii. The substrate is pre-heated to a temperature of 80° C. by        submerging the substrate in a water bath at 80° C.    -   iii. The substrate is then transferred to a 20% sulphuric acid        bath at 80° C. and left for 15 minutes. The substrate is then        rinsed in deionised water.    -   iv. The substrate is placed in a PdCl₂ solution (1 g L⁻¹) for 2        minutes. The substrate is then rinsed in deionised water.    -   v. The substrate is then re-heated to 80° C. as in (ii).    -   vi. At least 10 litres of nanoFLUX electroless deposition        solution as defined above are heated to 75° C. in a reservoir.        The electroless deposition solution is continuously pumped from        the reservoir over a surface of the substrate and returned to        the reservoir for a period of two hours.    -   vii. The coated substrate is rinsed in deionised water.    -   viii. The coated substrate is dried in an oven.

The above experimental protocol can be modified to produce a coatingcomprising clusters by adjusting step (vi) such that:

-   -   vi. At least 50 litres of nanoFLUX electroless deposition        solution are heated to 75° C. in a reservoir. The electroless        deposition solution is continuously pumped from the reservoir        over a surface of the substrate and returned to the reservoir        for a period of four hours.

-   3. Flow process for coating a flow channel of a heat exchanger

This flow process protocol is for coating a flow channel of a heatexchanger. The heat exchanger comprises a first flow channel having twoends (which is intended to be coated) and a second flow channel havingtwo ends (which is for carrying a heat transfer fluid). In thisprotocol, the flow channel to be coated has a surface area of one squaremetre.

-   -   i. The second flow channel of the substrate (i.e. the flow        channel that is not to be coated) is attached at each end to a        water source that is maintained at 80° C. Water is pumped        through the second flow channel continuously.    -   ii. The first flow channel of the substrate (i.e. the flow        channel that is to be coated) is attached at each end to a        reservoir of 20% sulphuric acid bath at 80° C. Sulphuric acid        from the reservoir is pumped through the first flow channel for        15 minutes.    -   iii. The pumping of water and acid is stopped.    -   iv. All acid is drained out of the first flow channel.    -   v. The first flow channel is then connected to a source of        deionised water. Deionised water is pumped through the channel        for 5 minutes or until the water exiting the first flow channel        runs clear. All water is then drained out of the first flow        channel.    -   vi. The first flow channel is connected at both ends to a source        of PdCl₂ solution (1 g L⁻¹) at room temperature. PdCl₂ solution        is pumped into the first flow channel and left for 2 minutes.        All PdCl₂ solution is then drained from the first flow channel.    -   vii. The substrate is then rinsed in deionised water as in step        (v).    -   viii. The pumping of water at 80° C. through the second flow        channel is restarted.    -   ix. The first flow channel is connected at both ends to a        reservoir containing at least 10 litres of nanoFLUX electroless        deposition solution (as defined above) maintained at 75° C. The        electroless deposition solution is continuously pumped from the        reservoir into the flow channel slowly, such that the first flow        channel is filled with the electroless deposition solution after        a period of five minutes or more (for instance 0.1 L min⁻¹ for a        heat exchanger having a volume of 1 L).    -   x. The pumping rate of electroless deposition solution is        increased by a factor of ten (for instance to 1.0 L min⁻¹). This        pumping rate is maintained for two hours.    -   xi. The pumping of electroless deposition solution is stopped        and all electroless deposition solution is drained out of the        first flow channel.    -   xii. The coated substrate is rinsed in deionised water as in        step (v).    -   xiii. The coated substrate is dried in an oven.

The above experimental protocol can be modified to produce a coatingcomprising clusters in the first flow channel by:

-   -   adjusting step (ix) such that the first flow channel is        connected at both ends to a reservoir containing at least 50        litres of nanoFLUX electroless deposition solution maintained at        75° C.; and    -   adjusting step (x) such that the higher pumping rate is        maintained for a period of four hours.

The skilled person will appreciate that the specific features of theabove processes, such as temperature and pump rate, can be varied.

EXAMPLES

-   1. Preparation of sample coatings.

Small test pieces of copper were coated according to the method of theinvention using the bath process protocol (protocol 1), above. Thecoating times were varied to achieve different spike lengths. At least100 ml of nanoFLUX solution was used in each case. The resulting heatexchange elements according to the invention were images using scanningelectron microscopy (SEM). The results are shown in FIG. 3 (ordinaryprotocol) and FIG. 5 (protocol modified to produce clusters).

FIG. 3(a) shows a coating comprising spikes of 1 to 3 μm in length,obtained by subjecting the test piece of copper to a bath of nanoFLUXsolution for up to 1 hour. FIG. 3(b) shows a coating comprising spikesof 4 to 5μm in length, obtained by subjecting the test piece of copperto a bath of nanoFLUX solution for approximately 2 hours. FIG. 3(c)shows a coating comprising spikes of 8 to 10 μm in length, subjectingthe test piece of copper to a bath of nanoFLUX solution for up to 4hours.

FIG. 5(a) shows a coating comprising spikes of approximately 7 μm inlength arranged in clusters. This coating was obtained by subjecting thetest piece of copper to a bath of nanoFLUX solution for approximately 3hours. This was reactivated and then coated for another 3 hours. FIG.5(b) also shows a coating of spikes arranged in clusters.

A test piece of steel wire mesh comprising wires 75 μm in diameter wasalso coated according to the method of the invention using the bathprocess protocol (protocol 1), above. The wire mesh was subjected to thebath of nanoFLUX solution for approximately 3 hours. The product wasimaged using SEM and the results are shown in FIG. 6.

-   2. Preparation of a heat exchanger.

A brazed plate heat exchanger made of 316 stainless steel with a copperbrazing, having a channel for a heat transfer fluid and a channel for arefrigerant, was coated according to the method of the invention usingthe flow process for coating a channel of a heat exchanger (protocol 3above). This produced a heat exchanger having a coating comprisingspikes approximately 3 μm in length along the inside of the channel fora refrigerant. An SEM image of the inside of that channel is shown inFIG. 4.

This example shows that the process of the invention may be used toretro-fit a heat exchange element of the invention into an existing heatexchanger. The process may be used to coat all or part of an existingheat exchanger to prepare a product according to the invention.

-   3. Efficiency of heat transfer of the heat exchange element in pool    boiling.

A heat exchange element was produced according to the process of theinvention, by coating a test piece of copper according to protocol 2,above, except that in step vi the test piece was placed in a bath ofnanoFLUX solution at 75° C. for up to 4 hours. The heat exchange elementwas similar to that shown in FIG. 3(c) (comprising spikes of 8 to 10 μmin length). It was tested for its ability to transfer heat to an organicrefrigerant in a pool boiling experiment. The pool boiling experiment isdescribed in “Compound effect of EHD and surface roughness in poolboiling and CHF with R-123”, Ahmad et al., Applied Thermal Engineering,vol. 31, pp. 1994-2003, 2011, and in “Pool boiling on Modified SurfacesUsing R-123”, Ahmad et al., Heat Transfer Engineering, Vol 35, Issue16-17, 2014. The results are shown in FIGS. 7 and 8.

FIG. 7 shows the heat flux in kW m⁻² from a surface to an organicrefrigerant as a function of wall superheat (ΔT_(c)). Results are givenfor a coated copper heat exchange element such as that shown in FIG.3(c) and for a polished oxygen-free copper surface. The results for theheat exchange element according to the invention are shown in blue andappear on the steeply-sloping line to the left of the graph. The resultsfor the polished surface are shown in black and appear on the flatterline along the bottom of the graph. FIG. 7 shows that the element of theinvention can achieve a high heat flux from element to refrigerant whilemaintaining a low superheat. Even when a high heat flux is provided tothe surface, the element transfers heat to the refrigerant soefficiently that its temperature does not rise far above that above therefrigerant. The surface worked so well that the heating units providingheat to the surface could not keep up with the rapid loss of heat fromthe surface. By contrast, the polished surface transfers heat to therefrigerant slowly. Heat flux to the surface heats the polished surfaceto a temperature high above that of the refrigerant as heat dissipatesout of the surface and into the refrigerant slowly.

FIG. 8 shows the heat flux in kW m⁻² from a surface to an organicrefrigerant as a function of wall superheat (ΔT_(c)) for variousdifferent surfaces:

-   -   (i) a polished surface (the black line with the flattest slope).    -   (ii) a surface coated with a copper coating such as that        described in WO2014/064450, comprising copper surface structures        of the order of 500 nm high (green line, almost flat up to a        ΔT_(c) of approximately 11° C. and then rising sharply).    -   (iii) a heat exchange element according to the invention        comprising a copper substrate coated with a coating comprising        copper spikes 1 μm long (red line, almost flat up to a ΔT_(c) of        approximately 8° C. and then rising sharply).    -   (iv) a heat exchange element according to the invention        comprising a copper substrate coated with a coating comprising        copper spikes 10 μm long (blue line, rising sharply from a        ΔT_(c) of approximately 2° C.).

The heat exchange elements of the invention produced a very high heatflux, up to the point where the test rig failed. Moreover, the heatexchange elements of the invention (particularly that having spikes 10μm long) maintained a very low superheat even at high heat flux,illustrating the excellent efficiency of the heat transfer from surfaceto refrigerant.

-   4. Efficiency of heat transfer of the heat exchange element in flow    boiling.

A heat exchange element according to the invention comprising a thinmetal tube as a substrate was prepared by the electroless flowdeposition process of the invention. A coating was provided on the innersurface of the tube such as the coating shown in FIG. 4. An organicrefrigerant was flowed through the tube whilst it was heated. The flowrate of the organic refrigerant was set to be 200 kg m⁻² s⁻¹, 300 kg m⁻²s⁻¹, 400 kg m⁻² s⁻¹, and 500 kg m⁻² s⁻¹ in turn. The heat transfercoefficient at each flow rate was measured. The heat transfercoefficient of an uncoated tube at each flow rate was also measured. Theexperimental protocol for measuring the heat transfer coefficient isdescribed in “Flow Boiling Heat Transfer In A Vertical Small-DiameterTube: Effect Of Different Fluids And Surface Characteristics”,Al-Gaheeshi et al., Conference: Proceedings of the 4th InternationalForum on Heat Transfer, IFHT2016, Nov. 2-4, 2016, at Sendai, Japan. Theheat transfer coefficient in W m⁻² K⁻¹ is a measure of a measure of theefficiency of heat transfer. A large heat transfer coefficient showsthat the heat exchange element transfers heat more efficiently.

The results are shown in FIG. 9. As can be seen in the Figure, thecoated tube had a higher heat transfer coefficient than the uncoatedtube at each flow rate. The coating to produce a heat exchange elementaccording to the invention therefore improved heat flux across thesurface.

-   5. Variation in deposition time.

The electroless deposition process of the invention can be adjusted tovary the size of the deposited structures. FIG. 10 shows the approximatespike height (dashed upper line) and spike radius at the base (solidlower line) achieved by varying the time for which a substrate issubjected to electroless deposition solution, e.g. to a flow ofelectroless deposition solution. The spike length increases with time,as does the base radius.

The spikes are approximately conical in shape. The length and radiusabove are obtained by approximating each spike to a cone whose axispasses through the cone's base at a right angle. The base of the cone isthe circle in the plane at the base of the cone's shortest side. Thus,spike length is the length of the approximate cone axis from base totip, and base radius is the radius of the circular base in thisapproximation.

It was noted while performing these experiments that the cone angle(that is, the angle the side of the cone makes with the base) is notaffected by the length of time for which electroless deposition isperformed. Thus, the process of the invention can produce long, sharpspikes; the spikes do not become less sharp as they become longer duringthe process of the invention.

-   6. Comparison of heat transfer efficiency of coated vs uncoated heat    exchanger

A coated evaporator (that is, a heat exchanger) was prepared accordingto Protocol 3 above. As explained in that protocol, the flow channel ofthe evaporator for receiving refrigerant was coated according to theprocess of the invention. However, the other flow channel of theevaporator for receiving water was not coated.

An uncoated evaporator was also obtained for comparative purposes. Thisuncoated evaporator was identical in structure to the other evaporator,but neither flow channel of the uncoated evaporator was coated inaccordance with the invention.

The coated and uncoated evaporators were incorporated in turn in a testrig. The test rig is shown in FIG. 11.

The system used as the working refrigerant R245fa. The ability of theevaporator to transfer heat from water to the refrigerant was thenevaluated. Refrigerant and water were circulated through the evaporatorsin accordance with normal usage of a heat exchanger. During theexperimental tests performed in this regard, the rate of heat transferfrom water to the refrigerant was assessed. Multiple experiments werecarried out to test the coating of the coated evaporator under differentflow rates. The experiments were repeated using the uncoated evaporatorbut the same flow parameters to enable a comparison.

Based on the experiments performed, the heat transfer coefficient (UA)was calculated. This is the proportionality constant between the heatflux across a surface and the temperature differential that existedacross the surface. Thus, where the surface has a large heat transfercoefficient, it can efficiently transfer heat even where the differencein temperature across the two sides of the surface is small. Surfaceswith smaller heat transfer coefficients require a larger difference intemperature across the surface (i.e. a larger driving force) before thesurface will permit an appreciable flow of heat Methods of calculatingthe heat transfer coefficient are given by Fernando et al. in “Propaneheat pump with low refrigerant charge: design and laboratory tests”,International Journal of Refrigeration, 27(7), pp.761-773, 2004, and byDutto et al. in “Performance of brazed plate heat exchanger set in heatpump Proceedings of the 18th International Congress of Refrigeration,new challenges in refrigeration”, Montreal, Quebec, Canada, vol. 3(10-17 Aug. 1991).

The heat transfer coefficient as a function of temperature for the twoevaporators is shown in FIG. 12. The refrigerant flow rate through theevaporators in both cases was 0.0121 kg/s.

The UA of the uncoated evaporator is shown by the square markers, and isin the region of 200 W/K. However, the coated evaporator, shown bycircular markers, achieves heat transfer coefficients in the region of300 W/K. For example, comparing the heat transfer coefficient calculatedat a rate of heat transfer on the water side of the evaporators of 2.25kW shows that the UA of the coated evaporator is 51.74% higher than thatof the uncoated evaporator. This is a remarkable improvement.

Moreover, this was achieved with very little pressure drop on therefrigerant-coated side of the evaporator during operation. The pressuredrop is the difference in flow pressure along the flow path (forexample, between the point of entry to the heat exchanger and the pointof exit). The pressure drop is caused by turbulence in the flow of fluidalong the flow path. A high pressure drop means that the pump pumpingfluid (e.g. refrigerant) through the heat exchanger needs to work harderto force the fluid through. It is therefore advantageous that a highpressure drop is avoided.

1. A heat exchange element comprising a substrate and a coating, whereinthe heat exchange element defines a flow path for flow of fluid, andwherein at least a part of the flow path is coated with the coating,wherein: the coating comprises a metal; the coating comprises aplurality of spikes having a length of up to 100 μm; the coatingcomprises a first region at an end of the flow path in which the averagespike length is S₁ and a second region on the flow path in which theaverage spike length is S₂; and S₁ is greater than S₂.
 2. A heatexchange element according to claim 1 wherein the spikes have a lengthof at least 1 μm and no more than 50 μm.
 3. A heat exchange elementaccording to claim 1 or wherein S₂ is from 50% to 90% of S₁.
 4. A heatexchange element according to claim 1 wherein S₁ is from 2 μm to 10 μm.5. A heat exchange element according to claim 1 wherein the spikes havea thickness at their tip of 100 nm or less.
 6. A heat exchange elementaccording to claim 1 wherein the said plurality of spikes are arrangedin one or more clusters, wherein each cluster comprises two or morespikes.
 7. A heat exchange element according to claim 6 wherein thediameter of each cluster is from 10 to 50 μm.
 8. A heat exchange elementaccording to claim 1 wherein the thickness of the coating is 10 μm ormore.
 9. A heat exchange element according to claim 1 wherein thecoating comprises copper.
 10. A heat exchange element according to claim1 wherein the coating comprises 80% metal by weight of the coating. 11.(canceled)
 12. A heat exchange element according to claim 1 wherein theaverage spike length is graduated along all or part of the flow path.13. A heat exchange element according to claim 1 wherein the coatingcomprises a surface layer on the coating.
 14. A heat exchange elementaccording to claim 1 wherein the substrate is a metal object.
 15. A heatexchange element according to claim 1 wherein the substrate is a heatexchanger suitable for transferring heat to a liquid.
 16. A heatexchange element according to claim 1 wherein the flow path comprises aflow channel and wherein the coating is present on at least a part ofthe surface of said flow channel.
 17. A heat exchange element accordingto claim 16 wherein the first region is located at or near to an inletto said flow channel and wherein the second region is located at agreater distance from the inlet than the first region.
 18. A heatexchange element according to claim 1 wherein the heat exchange elementcontains a refrigerant.
 19. A method of transferring heat to or from afluid which comprises providing the fluid to a flow path of a heatexchange element as defined in claim
 1. 20. (canceled)
 21. (canceled)22. (canceled)
 23. A process for producing a heat exchange elementcomprising a substrate and a coating, wherein: the coating comprises ametal; and flowing an electroless deposition solution over a surface ofthe substrate.
 24. A process according to claim 23 wherein the heatexchange element is as defined in of claim
 1. 25. A process according toclaim 23 wherein the process comprises: flowing the electrolessdeposition solution from a reservoir of electroless deposition solutionover the surface of the substrate; and returning the electrolessdeposition solution to the said reservoir.
 26. A process according toclaim 23 wherein the process comprises: flowing an electrolessdeposition solution over a surface of the substrate at a first flow rateF₁; and flowing an electroless deposition solution over the said surfaceof the substrate at a second flow rate F₂ optionally wherein F₂ isgreater than F₁.
 27. (canceled)
 28. A process according to claim 23wherein the process comprises pumping the electroless depositionsolution to cause the electroless deposition solution to flow over asurface of the substrate.
 29. A process according to claim 23 whereinthe substrate comprises a flow channel, and the process the processcomprises flowing an electroless deposition solution through said flowchannel.
 30. (canceled)
 31. A process according to claim 23 wherein theelectroless deposition solution is an aqueous solution.
 32. A processaccording to claim 23 wherein the electroless deposition comprisescopper and/or nickel ions.
 33. (canceled)
 34. (canceled)
 35. A processaccording to claim 23 wherein the process comprises providing theelectroless deposition solution to a surface of the substrate for a timeT, wherein T is the time taken for the electroless deposition solutionto become depleted by 5 to 50%.
 36. (canceled)
 37. (canceled)